Update iterators for consistency across library

This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

include/git2/attr.h

@@ -172,18 +172,17 @@ GIT_EXTERN(int) git_attr_get_many(
  *
  * @param repo The repository containing the path.
  * @param flags A combination of GIT_ATTR_CHECK... flags.
- * @param path The path inside the repo to check attributes.  This
- *             does not have to exist, but if it does not, then
- *             it will be treated as a plain file (i.e. not a directory).
- * @param callback The function that will be invoked on each attribute
- *             and attribute value.  The name parameter will be the name
- *             of the attribute and the value will be the value it is
- *             set to, including possibly NULL if the attribute is
- *             explicitly set to UNSPECIFIED using the ! sign.  This
- *             will be invoked only once per attribute name, even if
- *             there are multiple rules for a given file.  The highest
- *             priority rule will be used.
+ * @param path Path inside the repo to check attributes.  This does not have
+ *             to exist, but if it does not, then it will be treated as a
+ *             plain file (i.e. not a directory).
+ * @param callback Function to invoke on each attribute name and value.  The
+ *             value may be NULL is the attribute is explicitly set to
+ *             UNSPECIFIED using the '!' sign.  Callback will be invoked
+ *             only once per attribute name, even if there are multiple
+ *             rules for a given file.  The highest priority rule will be
+ *             used.  Return a non-zero value from this to stop looping.
  * @param payload Passed on as extra parameter to callback function.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_attr_foreach(
 	git_repository *repo,

include/git2/branch.h

@@ -74,6 +74,8 @@ GIT_EXTERN(int) git_branch_delete(
 /**
  * Loop over all the branches and issue a callback for each one.
  *
+ * If the callback returns a non-zero value, this will stop looping.
+ *
  * @param repo Repository where to find the branches.
  *
  * @param list_flags Filtering flags for the branch
@@ -84,7 +86,7 @@ GIT_EXTERN(int) git_branch_delete(
  *
  * @param payload Extra parameter to callback function.
  *
- * @return 0 or an error code.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_branch_foreach(
 		git_repository *repo,

include/git2/config.h

@@ -302,12 +302,12 @@ GIT_EXTERN(int) git_config_delete(git_config *cfg, const char *name);
  * The callback receives the normalized name and value of each variable
  * in the config backend, and the data pointer passed to this function.
  * As soon as one of the callback functions returns something other than 0,
- * this function returns that value.
+ * this function stops iterating and returns `GIT_EUSER`.
  *
  * @param cfg where to get the variables from
  * @param callback the function to call on each variable
  * @param payload the data to pass to the callback
- * @return 0 or the return value of the callback which didn't return 0
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_config_foreach(
 	git_config *cfg,

include/git2/diff.h

@@ -332,6 +332,9 @@ GIT_EXTERN(int) git_diff_merge(
  * callbacks will not be invoked for binary files on the diff list or for
  * files whose only changed is a file mode change.
  *
+ * Returning a non-zero value from any of the callbacks will terminate
+ * the iteration and cause this return `GIT_EUSER`.
+ *
  * @param diff A git_diff_list generated by one of the above functions.
  * @param cb_data Reference pointer that will be passed to your callbacks.
  * @param file_cb Callback function to make per file in the diff.
@@ -341,6 +344,7 @@ GIT_EXTERN(int) git_diff_merge(
  * @param line_cb Optional callback to make per line of diff text.  This
  *                same callback will be made for context lines, added, and
  *                removed lines, and even for a deleted trailing newline.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_foreach(
 	git_diff_list *diff,
@@ -351,6 +355,14 @@ GIT_EXTERN(int) git_diff_foreach(
 
 /**
  * Iterate over a diff generating text output like "git diff --name-status".
+ *
+ * Returning a non-zero value from the callbacks will terminate the
+ * iteration and cause this return `GIT_EUSER`.
+ *
+ * @param diff A git_diff_list generated by one of the above functions.
+ * @param cb_data Reference pointer that will be passed to your callback.
+ * @param print_cb Callback to make per line of diff text.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_print_compact(
 	git_diff_list *diff,
@@ -362,6 +374,9 @@ GIT_EXTERN(int) git_diff_print_compact(
  *
  * This is a super easy way to generate a patch from a diff.
  *
+ * Returning a non-zero value from the callbacks will terminate the
+ * iteration and cause this return `GIT_EUSER`.
+ *
  * @param diff A git_diff_list generated by one of the above functions.
  * @param cb_data Reference pointer that will be passed to your callbacks.
  * @param print_cb Callback function to output lines of the diff.  This
@@ -369,6 +384,7 @@ GIT_EXTERN(int) git_diff_print_compact(
  *                 headers, and diff lines.  Fortunately, you can probably
  *                 use various GIT_DIFF_LINE constants to determine what
  *                 text you are given.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_print_patch(
 	git_diff_list *diff,
@@ -393,6 +409,8 @@ GIT_EXTERN(int) git_diff_print_patch(
  * When at least one of the blobs being dealt with is binary, the
  * `git_diff_delta` binary attribute will be set to 1 and no call to the
  * hunk_cb nor line_cb will be made.
+ *
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_blobs(
 	git_blob *old_blob,

include/git2/errors.h

@@ -25,6 +25,7 @@ enum {
 	GIT_EEXISTS = -4,
 	GIT_EAMBIGUOUS = -5,
 	GIT_EBUFS = -6,
+	GIT_EUSER = -7,
 
 	GIT_PASSTHROUGH = -30,
 	GIT_REVWALKOVER = -31,

include/git2/notes.h

@@ -119,13 +119,15 @@ typedef struct {
  *
  * @param repo Repository where to find the notes.
  *
- * @param notes_ref OID reference to read from (optional); defaults to "refs/notes/commits".
+ * @param notes_ref OID reference to read from (optional); defaults to
+ *        "refs/notes/commits".
  *
- * @param note_cb Callback to invoke per found annotation.
+ * @param note_cb Callback to invoke per found annotation.  Return non-zero
+ *        to stop looping.
  *
  * @param payload Extra parameter to callback function.
  *
- * @return 0 or an error code.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_note_foreach(
 	git_repository *repo,

include/git2/odb.h

@@ -176,13 +176,14 @@ GIT_EXTERN(int) git_odb_exists(git_odb *db, const git_oid *id);
  * List all objects available in the database
  *
  * The callback will be called for each object available in the
- * database. Note that the objects are likely to be returned in the
- * index order, which would make accessing the objects in that order
- * inefficient.
+ * database. Note that the objects are likely to be returned in the index
+ * order, which would make accessing the objects in that order inefficient.
+ * Return a non-zero value from the callback to stop looping.
  *
  * @param db database to use
  * @param cb the callback to call for each object
  * @param data data to pass to the callback
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_odb_foreach(git_odb *db, int (*cb)(git_oid *oid, void *data), void *data);
 

include/git2/refs.h

@@ -268,14 +268,15 @@ GIT_EXTERN(int) git_reference_list(git_strarray *array, git_repository *repo, un
  *
  * The `callback` function will be called for each of the references
  * in the repository, and will receive the name of the reference and
- * the `payload` value passed to this method.
+ * the `payload` value passed to this method.  Returning a non-zero
+ * value from the callback will terminate the iteration.
  *
  * @param repo Repository where to find the refs
  * @param list_flags Filtering flags for the reference
  *		listing.
  * @param callback Function which will be called for every listed ref
  * @param payload Additional data to pass to the callback
- * @return 0 or an error code
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_reference_foreach(git_repository *repo, unsigned int list_flags, int (*callback)(const char *, void *), void *payload);
 

include/git2/remote.h

@@ -133,9 +133,12 @@ GIT_EXTERN(int) git_remote_connect(git_remote *remote, int direction);
  * The remote (or more exactly its transport) must be connected. The
  * memory belongs to the remote.
  *
+ * If you a return a non-zero value from the callback, this will stop
+ * looping over the refs.
+ *
  * @param refs where to store the refs
  * @param remote the remote
- * @return 0 or an error code
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_remote_ls(git_remote *remote, git_headlist_cb list_cb, void *payload);
 

include/git2/status.h

@@ -38,11 +38,11 @@ enum {
  *
  * The callback is passed the path of the file, the status and the data
  * pointer passed to this function. If the callback returns something other
- * than 0, this function will return that value.
+ * than 0, this function will stop looping and return GIT_EUSER.
  *
  * @param repo a repository object
  * @param callback the function to call on each file
- * @return 0 on success or the return value of the callback that was non-zero
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_status_foreach(
 	git_repository *repo,

src/attr.c

@@ -163,14 +163,17 @@ int git_attr_foreach(
 					continue;
 
 				git_strmap_insert(seen, assign->name, assign, error);
-				if (error >= 0)
-					error = callback(assign->name, assign->value, payload);
+				if (error < 0)
+					goto cleanup;
 
-				if (error != 0)
+				error = callback(assign->name, assign->value, payload);
+				if (error) {
+					error = GIT_EUSER;
 					goto cleanup;
 				}
 			}
 		}
+	}
 
 cleanup:
 	git_strmap_free(seen);

src/config_file.c

@@ -218,9 +218,11 @@ static int file_foreach(
 				continue;
 
 			/* abort iterator on non-zero return value */
-			if ((result = fn(key, var->value, data)) != 0)
+			if (fn(key, var->value, data)) {
+				result = GIT_EUSER;
 				goto cleanup;
 			}
+		}
 	);
 
 cleanup:

src/diff_output.c

@@ -23,6 +23,7 @@ typedef struct {
 	unsigned int index;
 	git_diff_delta *delta;
 	git_diff_range range;
+	int error;
 } diff_output_info;
 
 static int read_next_int(const char **str, int *value)
@@ -49,25 +50,24 @@ static int diff_output_cb(void *priv, mmbuffer_t *bufs, int len)
 
 		/* expect something of the form "@@ -%d[,%d] +%d[,%d] @@" */
 		if (*scan != '@')
-			return -1;
-
-		if (read_next_int(&scan, &range.old_start) < 0)
-			return -1;
-		if (*scan == ',' && read_next_int(&scan, &range.old_lines) < 0)
-			return -1;
-
-		if (read_next_int(&scan, &range.new_start) < 0)
-			return -1;
-		if (*scan == ',' && read_next_int(&scan, &range.new_lines) < 0)
-			return -1;
-
-		if (range.old_start < 0 || range.new_start < 0)
-			return -1;
-
+			info->error = -1;
+		else if (read_next_int(&scan, &range.old_start) < 0)
+			info->error = -1;
+		else if (*scan == ',' && read_next_int(&scan, &range.old_lines) < 0)
+			info->error = -1;
+		else if (read_next_int(&scan, &range.new_start) < 0)
+			info->error = -1;
+		else if (*scan == ',' && read_next_int(&scan, &range.new_lines) < 0)
+			info->error = -1;
+		else if (range.old_start < 0 || range.new_start < 0)
+			info->error = -1;
+		else {
 			memcpy(&info->range, &range, sizeof(git_diff_range));
 
-		return info->hunk_cb(
-			info->cb_data, info->delta, &range, bufs[0].ptr, bufs[0].size);
+			if (info->hunk_cb(
+					info->cb_data, info->delta, &range, bufs[0].ptr, bufs[0].size))
+				info->error = GIT_EUSER;
+		}
 	}
 
 	if ((len == 2 || len == 3) && info->line_cb) {
@@ -80,23 +80,24 @@ static int diff_output_cb(void *priv, mmbuffer_t *bufs, int len)
 			GIT_DIFF_LINE_CONTEXT;
 
 		if (info->line_cb(
-			info->cb_data, info->delta, &info->range, origin, bufs[1].ptr, bufs[1].size) < 0)
-			return -1;
+				info->cb_data, info->delta, &info->range, origin, bufs[1].ptr, bufs[1].size))
+			info->error = GIT_EUSER;
 
 		/* This should only happen if we are adding a line that does not
 		 * have a newline at the end and the old code did.  In that case,
 		 * we have a ADD with a DEL_EOFNL as a pair.
 		 */
-		if (len == 3) {
+		else if (len == 3) {
 			origin = (origin == GIT_DIFF_LINE_ADDITION) ?
 				GIT_DIFF_LINE_DEL_EOFNL : GIT_DIFF_LINE_ADD_EOFNL;
 
-			return info->line_cb(
-				info->cb_data, info->delta, &info->range, origin, bufs[2].ptr, bufs[2].size);
+			if (info->line_cb(
+					info->cb_data, info->delta, &info->range, origin, bufs[2].ptr, bufs[2].size))
+				info->error = GIT_EUSER;
 		}
 	}
 
-	return 0;
+	return info->error;
 }
 
 #define BINARY_DIFF_FLAGS (GIT_DIFF_FILE_BINARY|GIT_DIFF_FILE_NOT_BINARY)
@@ -318,6 +319,7 @@ int git_diff_foreach(
 	xdemitconf_t xdiff_config;
 	xdemitcb_t   xdiff_callback;
 
+	memset(&info, 0, sizeof(info));
 	info.diff    = diff;
 	info.cb_data = data;
 	info.hunk_cb = hunk_cb;
@@ -422,10 +424,10 @@ int git_diff_foreach(
 		 * diffs to tell if a file has really been changed.
 		 */
 
-		if (file_cb != NULL) {
-			error = file_cb(
-				data, delta, (float)info.index / diff->deltas.length);
-			if (error < 0)
+		if (file_cb != NULL &&
+			file_cb(data, delta, (float)info.index / diff->deltas.length))
+		{
+			error = GIT_EUSER;
 			goto cleanup;
 		}
 
@@ -451,6 +453,7 @@ int git_diff_foreach(
 
 		xdl_diff(&old_xdiff_data, &new_xdiff_data,
 			&xdiff_params, &xdiff_config, &xdiff_callback);
+		error = info.error;
 
 cleanup:
 		release_content(&delta->old_file, &old_data, old_blob);
@@ -524,7 +527,11 @@ static int print_compact(void *data, git_diff_delta *delta, float progress)
 	if (git_buf_oom(pi->buf))
 		return -1;
 
-	return pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_FILE_HDR, git_buf_cstr(pi->buf), git_buf_len(pi->buf));
+	if (pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_FILE_HDR,
+			git_buf_cstr(pi->buf), git_buf_len(pi->buf)))
+		return GIT_EUSER;
+
+	return 0;
 }
 
 int git_diff_print_compact(
@@ -586,7 +593,6 @@ static int print_patch_file(void *data, git_diff_delta *delta, float progress)
 	const char *oldpath = delta->old_file.path;
 	const char *newpfx = pi->diff->opts.new_prefix;
 	const char *newpath = delta->new_file.path;
-	int result;
 
 	GIT_UNUSED(progress);
 
@@ -619,9 +625,8 @@ static int print_patch_file(void *data, git_diff_delta *delta, float progress)
 	if (git_buf_oom(pi->buf))
 		return -1;
 
-    result = pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_FILE_HDR, git_buf_cstr(pi->buf), git_buf_len(pi->buf));
-    if (result < 0)
-        return result;
+    if (pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_FILE_HDR, git_buf_cstr(pi->buf), git_buf_len(pi->buf)))
+		return GIT_EUSER;
 
     if (delta->binary != 1)
         return 0;
@@ -633,7 +638,11 @@ static int print_patch_file(void *data, git_diff_delta *delta, float progress)
 	if (git_buf_oom(pi->buf))
 		return -1;
 
-	return pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_BINARY, git_buf_cstr(pi->buf), git_buf_len(pi->buf));
+	if (pi->print_cb(pi->cb_data, delta, NULL, GIT_DIFF_LINE_BINARY,
+			git_buf_cstr(pi->buf), git_buf_len(pi->buf)))
+		return GIT_EUSER;
+
+	return 0;
 }
 
 static int print_patch_hunk(
@@ -649,7 +658,11 @@ static int print_patch_hunk(
 	if (git_buf_printf(pi->buf, "%.*s", (int)header_len, header) < 0)
 		return -1;
 
-	return pi->print_cb(pi->cb_data, d, r, GIT_DIFF_LINE_HUNK_HDR, git_buf_cstr(pi->buf), git_buf_len(pi->buf));
+	if (pi->print_cb(pi->cb_data, d, r, GIT_DIFF_LINE_HUNK_HDR,
+			git_buf_cstr(pi->buf), git_buf_len(pi->buf)))
+		return GIT_EUSER;
+
+	return 0;
 }
 
 static int print_patch_line(
@@ -674,7 +687,11 @@ static int print_patch_line(
 	if (git_buf_oom(pi->buf))
 		return -1;
 
-	return pi->print_cb(pi->cb_data, delta, range, line_origin, git_buf_cstr(pi->buf), git_buf_len(pi->buf));
+	if (pi->print_cb(pi->cb_data, delta, range, line_origin,
+			git_buf_cstr(pi->buf), git_buf_len(pi->buf)))
+		return GIT_EUSER;
+
+	return 0;
 }
 
 int git_diff_print_patch(
@@ -763,11 +780,8 @@ int git_diff_blobs(
 	if (file_is_binary_by_content(&delta, &old_map, &new_map) < 0)
 		return -1;
 
-	if (file_cb != NULL) {
-		int error = file_cb(cb_data, &delta, 1);
-		if (error < 0)
-			return error;
-	}
+	if (file_cb != NULL && file_cb(cb_data, &delta, 1))
+		return GIT_EUSER;
 
 	/* don't do hunk and line diffs if the two blobs are identical */
 	if (delta.status == GIT_DELTA_UNMODIFIED)
@@ -777,6 +791,7 @@ int git_diff_blobs(
 	if (delta.binary == 1)
 		return 0;
 
+	memset(&info, 0, sizeof(info));
 	info.diff    = NULL;
 	info.delta   = &delta;
 	info.cb_data = cb_data;
@@ -790,5 +805,5 @@ int git_diff_blobs(
 
 	xdl_diff(&old_data, &new_data, &xdiff_params, &xdiff_config, &xdiff_callback);
 
-	return 0;
+	return info.error;
 }

src/notes.c

@@ -522,11 +522,11 @@ static int process_entry_path(
 	int (*note_cb)(git_note_data *note_data, void *payload),
 	void *payload)
 {
-	int i = 0, j = 0, error = -1, len;
+	int i = 0, j = 0, error, len;
 	git_buf buf = GIT_BUF_INIT;
 	git_note_data note_data;
 
-	if (git_buf_puts(&buf, entry_path) < 0)
+	if ((error = git_buf_puts(&buf, entry_path)) < 0)
 		goto cleanup;
 
 	len = git_buf_len(&buf);
@@ -539,7 +539,6 @@ static int process_entry_path(
 
 		if (git__fromhex(buf.ptr[i]) < 0) {
 			/* This is not a note entry */
-			error = 0;
 			goto cleanup;
 		}
 
@@ -555,16 +554,17 @@ static int process_entry_path(
 
 	if (j != GIT_OID_HEXSZ) {
 		/* This is not a note entry */
-		error = 0;
 		goto cleanup;
 	}
 
-	if (git_oid_fromstr(&note_data.annotated_object_oid, buf.ptr) < 0)
-		return -1;
+	if ((error = git_oid_fromstr(
+			&note_data.annotated_object_oid, buf.ptr)) < 0)
+		goto cleanup;
 
 	git_oid_cpy(&note_data.blob_oid, note_oid);
 
-	error = note_cb(&note_data, payload);
+	if (note_cb(&note_data, payload))
+		error = GIT_EUSER;
 
 cleanup:
 	git_buf_free(&buf);
@@ -577,34 +577,27 @@ int git_note_foreach(
 	int (*note_cb)(git_note_data *note_data, void *payload),
 	void *payload)
 {
-	int error = -1;
+	int error;
 	git_iterator *iter = NULL;
 	git_tree *tree = NULL;
 	git_commit *commit = NULL;
 	const git_index_entry *item;
 
-	if ((error = retrieve_note_tree_and_commit(&tree, &commit, repo, &notes_ref)) < 0)
-		goto cleanup;
+	if (!(error = retrieve_note_tree_and_commit(
+			&tree, &commit, repo, &notes_ref)) &&
+		!(error = git_iterator_for_tree(&iter, repo, tree)))
+		error = git_iterator_current(iter, &item);
 
-	if (git_iterator_for_tree(&iter, repo, tree) < 0)
-		goto cleanup;
+	while (!error && item) {
+		error = process_entry_path(item->path, &item->oid, note_cb, payload);
 
-	if (git_iterator_current(iter, &item) < 0)
-		goto cleanup;
-
-	while (item) {
-		if (process_entry_path(item->path, &item->oid, note_cb, payload) < 0)
-			goto cleanup;
-
-		if (git_iterator_advance(iter, &item) < 0)
-			goto cleanup;
+		if (!error)
+			error = git_iterator_advance(iter, &item);
 	}
 
-	error = 0;
-
-cleanup:
 	git_iterator_free(iter);
 	git_tree_free(tree);
 	git_commit_free(commit);
+
 	return error;
 }

src/odb.c

@@ -609,9 +609,12 @@ int git_odb_foreach(git_odb *db, int (*cb)(git_oid *oid, void *data), void *data
 {
 	unsigned int i;
 	backend_internal *internal;
+
 	git_vector_foreach(&db->backends, i, internal) {
 		git_odb_backend *b = internal->backend;
-		b->foreach(b, cb, data);
+		int error = b->foreach(b, cb, data);
+		if (error < 0)
+			return error;
 	}
 
 	return 0;

src/odb_loose.c

@@ -680,6 +680,7 @@ struct foreach_state {
 	size_t dir_len;
 	int (*cb)(git_oid *oid, void *data);
 	void *data;
+	int cb_error;
 };
 
 GIT_INLINE(int) filename_to_oid(git_oid *oid, const char *ptr)
@@ -718,8 +719,10 @@ static int foreach_object_dir_cb(void *_state, git_buf *path)
 	if (filename_to_oid(&oid, path->ptr + state->dir_len) < 0)
 		return 0;
 
-	if (state->cb(&oid, state->data) < 0)
+	if (state->cb(&oid, state->data)) {
+		state->cb_error = GIT_EUSER;
 		return -1;
+	}
 
 	return 0;
 }
@@ -728,10 +731,7 @@ static int foreach_cb(void *_state, git_buf *path)
 {
 	struct foreach_state *state = (struct foreach_state *) _state;
 
-	if (git_path_direach(path, foreach_object_dir_cb, state) < 0)
-		return -1;
-
-	return 0;
+	return git_path_direach(path, foreach_object_dir_cb, state);
 }
 
 static int loose_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *oid, void *data), void *data)
@@ -749,14 +749,16 @@ static int loose_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *
 	git_buf_sets(&buf, objects_dir);
 	git_path_to_dir(&buf);
 
+	memset(&state, 0, sizeof(state));
 	state.cb = cb;
 	state.data = data;
 	state.dir_len = git_buf_len(&buf);
 
 	error = git_path_direach(&buf, foreach_cb, &state);
+
 	git_buf_free(&buf);
 
-	return error;
+	return state.cb_error ? state.cb_error : error;
 }
 
 static int loose_backend__stream_fwrite(git_oid *oid, git_odb_stream *_stream)

src/odb_pack.c

@@ -422,6 +422,7 @@ static int pack_backend__exists(git_odb_backend *backend, const git_oid *oid)
 
 static int pack_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *oid, void *data), void *data)
 {
+	int error;
 	struct git_pack_file *p;
 	struct pack_backend *backend;
 	unsigned int i;
@@ -430,12 +431,14 @@ static int pack_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *o
 	backend = (struct pack_backend *)_backend;
 
 	/* Make sure we know about the packfiles */
-	if (packfile_refresh_all(backend) < 0)
-		return -1;
+	if ((error = packfile_refresh_all(backend)) < 0)
+		return error;
 
 	git_vector_foreach(&backend->packs, i, p) {
-		git_pack_foreach_entry(p, cb, &data);
+		if ((error = git_pack_foreach_entry(p, cb, &data)) < 0)
+			return error;
 	}
+
 	return 0;
 }
 

src/pack.c

@@ -690,7 +690,6 @@ int git_pack_foreach_entry(
 	struct git_pack_file *p,
 	int (*cb)(git_oid *oid, void *data),
 	void *data)
-
 {
 	const unsigned char *index = p->index_map.data, *current;
 	unsigned stride;
@@ -722,7 +721,9 @@ int git_pack_foreach_entry(
 
 	current = index;
 	for (i = 0; i < p->num_objects; i++) {
-		cb((git_oid *)current, data);
+		if (cb((git_oid *)current, data))
+			return GIT_EUSER;
+
 		current += stride;
 	}
 

src/path.h

@@ -217,6 +217,7 @@ extern int git_path_apply_relative(git_buf *target, const char *relpath);
  *		the input state and the second arg is pathbuf. The function
  *		may modify the pathbuf, but only by appending new text.
  * @param state to pass to fn as the first arg.
+ * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 extern int git_path_direach(
 	git_buf *pathbuf,

src/refs.c

@@ -501,6 +501,7 @@ struct dirent_list_data {
 
 	int (*callback)(const char *, void *);
 	void *callback_payload;
+	int callback_error;
 };
 
 static int _dirent_loose_listall(void *_data, git_buf *full_path)
@@ -521,7 +522,10 @@ static int _dirent_loose_listall(void *_data, git_buf *full_path)
 			return 0; /* we are filtering out this reference */
 	}
 
-	return data->callback(file_path, data->callback_payload);
+	if (data->callback(file_path, data->callback_payload))
+		data->callback_error = GIT_EUSER;
+
+	return data->callback_error;
 }
 
 static int _dirent_loose_load(void *data, git_buf *full_path)
@@ -844,15 +848,17 @@ static int reference_path_available(
 	const char *ref,
 	const char* old_ref)
 {
+	int error;
 	struct reference_available_t data;
 
 	data.new_ref = ref;
 	data.old_ref = old_ref;
 	data.available = 1;
 
-	if (git_reference_foreach(repo, GIT_REF_LISTALL,
-		_reference_available_cb, (void *)&data) < 0)
-		return -1;
+	error = git_reference_foreach(
+		repo, GIT_REF_LISTALL, _reference_available_cb, (void *)&data);
+	if (error < 0)
+		return error;
 
 	if (!data.available) {
 		giterr_set(GITERR_REFERENCE,
@@ -1487,8 +1493,8 @@ int git_reference_foreach(
 			return -1;
 
 		git_strmap_foreach(repo->references.packfile, ref_name, ref, {
-			if (callback(ref_name, payload) < 0)
-				return 0;
+			if (callback(ref_name, payload))
+				return GIT_EUSER;
 		});
 	}
 
@@ -1500,14 +1506,16 @@ int git_reference_foreach(
 	data.repo = repo;
 	data.callback = callback;
 	data.callback_payload = payload;
+	data.callback_error = 0;
 
 	if (git_buf_joinpath(&refs_path, repo->path_repository, GIT_REFS_DIR) < 0)
 		return -1;
 
 	result = git_path_direach(&refs_path, _dirent_loose_listall, &data);
+
 	git_buf_free(&refs_path);
 
-	return result;
+	return data.callback_error ? GIT_EUSER : result;
 }
 
 static int cb__reflist_add(const char *ref, void *data)

src/status.c

@@ -114,7 +114,8 @@ int git_status_foreach_ext(
 	if (show == GIT_STATUS_SHOW_INDEX_THEN_WORKDIR) {
 		for (i = 0; !err && i < idx2head->deltas.length; i++) {
 			i2h = GIT_VECTOR_GET(&idx2head->deltas, i);
-			err = cb(i2h->old_file.path, index_delta2status(i2h->status), cbdata);
+			if (cb(i2h->old_file.path, index_delta2status(i2h->status), cbdata))
+				err = GIT_EUSER;
 		}
 		git_diff_list_free(idx2head);
 		idx2head = NULL;
@@ -130,14 +131,17 @@ int git_status_foreach_ext(
 		cmp = !w2i ? -1 : !i2h ? 1 : strcmp(i2h->old_file.path, w2i->old_file.path);
 
 		if (cmp < 0) {
-			err = cb(i2h->old_file.path, index_delta2status(i2h->status), cbdata);
+			if (cb(i2h->old_file.path, index_delta2status(i2h->status), cbdata))
+				err = GIT_EUSER;
 			i++;
 		} else if (cmp > 0) {
-			err = cb(w2i->old_file.path, workdir_delta2status(w2i->status), cbdata);
+			if (cb(w2i->old_file.path, workdir_delta2status(w2i->status), cbdata))
+				err = GIT_EUSER;
 			j++;
 		} else {
-			err = cb(i2h->old_file.path, index_delta2status(i2h->status) |
-					 workdir_delta2status(w2i->status), cbdata);
+			if (cb(i2h->old_file.path, index_delta2status(i2h->status) |
+				   workdir_delta2status(w2i->status), cbdata))
+				err = GIT_EUSER;
 			i++; j++;
 		}
 	}
@@ -146,6 +150,7 @@ cleanup:
 	git_tree_free(head);
 	git_diff_list_free(idx2head);
 	git_diff_list_free(wd2idx);
+
 	return err;
 }
 
@@ -166,9 +171,10 @@ int git_status_foreach(
 }
 
 struct status_file_info {
+	char *expected;
 	unsigned int count;
 	unsigned int status;
-	char *expected;
+	int ambiguous;
 };
 
 static int get_one_status(const char *path, unsigned int status, void *data)
@@ -183,6 +189,7 @@ static int get_one_status(const char *path, unsigned int status, void *data)
 		p_fnmatch(sfi->expected, path, 0) != 0)) {
 		giterr_set(GITERR_INVALID,
 			"Ambiguous path '%s' given to git_status_file", sfi->expected);
+		sfi->ambiguous = true;
 		return GIT_EAMBIGUOUS;
 	}
 
@@ -215,6 +222,9 @@ int git_status_file(
 
 	error = git_status_foreach_ext(repo, &opts, get_one_status, &sfi);
 
+	if (error < 0 && sfi.ambiguous)
+		error = GIT_EAMBIGUOUS;
+
 	if (!error && !sfi.count) {
 		giterr_set(GITERR_INVALID,
 			"Attempt to get status of nonexistent file '%s'", path);

src/transports/git.c

@@ -239,10 +239,8 @@ static int git_ls(git_transport *transport, git_headlist_cb list_cb, void *opaqu
 
 		pkt = (git_pkt_ref *)p;
 
-		if (list_cb(&pkt->head, opaque) < 0) {
-			giterr_set(GITERR_NET, "User callback returned error");
-			return -1;
-		}
+		if (list_cb(&pkt->head, opaque))
+			return GIT_EUSER;
 	}
 
 	return 0;

src/transports/http.c

@@ -324,10 +324,8 @@ static int http_ls(git_transport *transport, git_headlist_cb list_cb, void *opaq
 		if (p->type != GIT_PKT_REF)
 			continue;
 
-		if (list_cb(&p->head, opaque) < 0) {
-			giterr_set(GITERR_NET, "The user callback returned error");
-			return -1;
-		}
+		if (list_cb(&p->head, opaque))
+			return GIT_EUSER;
 	}
 
 	return 0;

src/transports/local.c

@@ -126,8 +126,8 @@ static int local_ls(git_transport *transport, git_headlist_cb list_cb, void *pay
 	assert(transport && transport->connected);
 
 	git_vector_foreach(refs, i, h) {
-		if (list_cb(h, payload) < 0)
-			return -1;
+		if (list_cb(h, payload))
+			return GIT_EUSER;
 	}
 
 	return 0;

tests-clar/attr/repo.c

@@ -113,6 +113,22 @@ static int count_attrs(
 	return 0;
 }
 
+static int cancel_iteration(
+	const char *name,
+	const char *value,
+	void *payload)
+{
+	GIT_UNUSED(name);
+	GIT_UNUSED(value);
+
+	*((int *)payload) -= 1;
+
+	if (*((int *)payload) < 0)
+		return -1;
+
+	return 0;
+}
+
 void test_attr_repo__foreach(void)
 {
 	int count;
@@ -131,6 +147,12 @@ void test_attr_repo__foreach(void)
 	cl_git_pass(git_attr_foreach(g_repo, 0, "sub/subdir_test2.txt",
 		&count_attrs, &count));
 	cl_assert(count == 6); /* repoattr, rootattr, subattr, reposub, negattr, another */
+
+	count = 2;
+	cl_assert_equal_i(
+		GIT_EUSER, git_attr_foreach(
+			g_repo, 0, "sub/subdir_test1", &cancel_iteration, &count)
+	);
 }
 
 void test_attr_repo__manpage_example(void)

tests-clar/config/read.c

@@ -226,7 +226,7 @@ void test_config_read__foreach(void)
 
 	count = 3;
 	cl_git_fail(ret = git_config_foreach(cfg, cfg_callback_countdown, &count));
-	cl_assert_equal_i(-100, ret);
+	cl_assert_equal_i(GIT_EUSER, ret);
 
 	git_config_free(cfg);
 }

tests-clar/diff/index.c

@@ -90,3 +90,53 @@ void test_diff_index__0(void)
 	git_tree_free(a);
 	git_tree_free(b);
 }
+
+static int diff_stop_after_2_files(
+	void *cb_data,
+	git_diff_delta *delta,
+	float progress)
+{
+	diff_expects *e = cb_data;
+
+	GIT_UNUSED(progress);
+	GIT_UNUSED(delta);
+
+	e->files++;
+
+	return (e->files == 2);
+}
+
+void test_diff_index__1(void)
+{
+	/* grabbed a couple of commit oids from the history of the attr repo */
+	const char *a_commit = "26a125ee1bf"; /* the current HEAD */
+	const char *b_commit = "0017bd4ab1ec3"; /* the start */
+	git_tree *a = resolve_commit_oid_to_tree(g_repo, a_commit);
+	git_tree *b = resolve_commit_oid_to_tree(g_repo, b_commit);
+	git_diff_options opts = {0};
+	git_diff_list *diff = NULL;
+	diff_expects exp;
+
+	cl_assert(a);
+	cl_assert(b);
+
+	opts.context_lines = 1;
+	opts.interhunk_lines = 1;
+
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_tree(g_repo, &opts, a, &diff));
+
+	cl_assert_equal_i(
+		GIT_EUSER,
+		git_diff_foreach(diff, &exp, diff_stop_after_2_files, NULL, NULL)
+	);
+
+	cl_assert(exp.files == 2);
+
+	git_diff_list_free(diff);
+	diff = NULL;
+
+	git_tree_free(a);
+	git_tree_free(b);
+}

tests-clar/notes/notes.c

@@ -95,11 +95,39 @@ void test_notes_notes__can_retrieve_a_list_of_notes_for_a_given_namespace(void)
 	create_note(&note_oid3, "refs/notes/i-can-see-dead-notes", "9fd738e8f7967c078dceed8190330fc8648ee56a", "I decorate 9fd7 and 4a20\n");
 	create_note(&note_oid4, "refs/notes/i-can-see-dead-notes", "4a202b346bb0fb0db7eff3cffeb3c70babbd2045", "I decorate 9fd7 and 4a20\n");
 
-	cl_git_pass(git_note_foreach(_repo, "refs/notes/i-can-see-dead-notes", note_list_cb, &retrieved_notes));
+	cl_git_pass(git_note_foreach
+(_repo, "refs/notes/i-can-see-dead-notes", note_list_cb, &retrieved_notes));
 
 	cl_assert_equal_i(4, retrieved_notes);
 }
 
+static int note_cancel_cb(git_note_data *note_data, void *payload)
+{
+	unsigned int *count = (unsigned int *)payload;
+
+	GIT_UNUSED(note_data);
+
+	(*count)++;
+
+	return (*count > 2);
+}
+
+void test_notes_notes__can_cancel_foreach(void)
+{
+	git_oid note_oid1, note_oid2, note_oid3, note_oid4;
+	unsigned int retrieved_notes = 0;
+
+	create_note(&note_oid1, "refs/notes/i-can-see-dead-notes", "a65fedf39aefe402d3bb6e24df4d4f5fe4547750", "I decorate a65f\n");
+	create_note(&note_oid2, "refs/notes/i-can-see-dead-notes", "c47800c7266a2be04c571c04d5a6614691ea99bd", "I decorate c478\n");
+	create_note(&note_oid3, "refs/notes/i-can-see-dead-notes", "9fd738e8f7967c078dceed8190330fc8648ee56a", "I decorate 9fd7 and 4a20\n");
+	create_note(&note_oid4, "refs/notes/i-can-see-dead-notes", "4a202b346bb0fb0db7eff3cffeb3c70babbd2045", "I decorate 9fd7 and 4a20\n");
+
+	cl_assert_equal_i(
+		GIT_EUSER,
+		git_note_foreach(_repo, "refs/notes/i-can-see-dead-notes",
+			note_cancel_cb, &retrieved_notes));
+}
+
 void test_notes_notes__retrieving_a_list_of_notes_for_an_unknown_namespace_returns_ENOTFOUND(void)
 {
 	int error;

tests-clar/odb/foreach.c

@@ -31,6 +31,24 @@ static int foreach_cb(git_oid *oid, void *data)
 
 void test_odb_foreach__foreach(void)
 {
+	nobj = 0;
 	cl_git_pass(git_odb_foreach(_odb, foreach_cb, NULL));
 	cl_assert(nobj == 1683);
 }
+
+static int foreach_stop_cb(git_oid *oid, void *data)
+{
+	GIT_UNUSED(data);
+	GIT_UNUSED(oid);
+
+	nobj++;
+
+	return (nobj == 1000);
+}
+
+void test_odb_foreach__interrupt_foreach(void)
+{
+	nobj = 0;
+	cl_assert_equal_i(GIT_EUSER, git_odb_foreach(_odb, foreach_stop_cb, NULL));
+	cl_assert(nobj == 1000);
+}

tests-clar/refs/branches/foreach.c

@@ -126,3 +126,28 @@ void test_refs_branches_foreach__retrieve_remote_symbolic_HEAD_when_present(void
 	assert_branch_has_been_found(exp, "nulltoken/HEAD");
 	assert_branch_has_been_found(exp, "nulltoken/HEAD");
 }
+
+static int branch_list_interrupt_cb(
+	const char *branch_name, git_branch_t branch_type, void *payload)
+{
+	int *count;
+
+	GIT_UNUSED(branch_type);
+	GIT_UNUSED(branch_name);
+
+	count = (int *)payload;
+	(*count)++;
+
+	return (*count == 5);
+}
+
+void test_refs_branches_foreach__can_cancel(void)
+{
+	int count = 0;
+
+	cl_assert_equal_i(GIT_EUSER,
+		git_branch_foreach(repo, GIT_BRANCH_LOCAL | GIT_BRANCH_REMOTE,
+			branch_list_interrupt_cb, &count));
+
+	cl_assert_equal_i(5, count);
+}

tests-clar/refs/foreachglob.c

@@ -68,3 +68,25 @@ void test_refs_foreachglob__retrieve_partially_named_references(void)
 
 	assert_retrieval("*test*", GIT_REF_LISTALL, 4);
 }
+
+
+static int interrupt_cb(const char *reference_name, void *payload)
+{
+	int *count = (int *)payload;
+
+	GIT_UNUSED(reference_name);
+
+	(*count)++;
+
+	return (*count == 11);
+}
+
+void test_refs_foreachglob__can_cancel(void)
+{
+	int count = 0;
+
+	cl_assert_equal_i(GIT_EUSER, git_reference_foreach_glob(
+		repo, "*", GIT_REF_LISTALL, interrupt_cb, &count) );
+
+	cl_assert_equal_i(11, count);
+}

tests-clar/status/worktree.c

@@ -591,7 +591,7 @@ void test_status_worktree__bracket_in_filename(void)
 
 	error = git_status_file(&status_flags, repo, FILE_WITH_BRACKET);
 	cl_git_fail(error);
-	cl_assert(error == GIT_EAMBIGUOUS);
+	cl_assert_equal_i(GIT_EAMBIGUOUS, error);
 
 	git_index_free(index);
 	git_repository_free(repo);
@@ -772,3 +772,28 @@ void test_status_worktree__disable_pathspec_match(void)
 
 	git_repository_free(repo);
 }
+
+
+static int cb_status__interrupt(const char *p, unsigned int s, void *payload)
+{
+	volatile int *count = (int *)payload;
+
+	GIT_UNUSED(p);
+	GIT_UNUSED(s);
+
+	(*count)++;
+
+	return (*count == 8);
+}
+
+void test_status_worktree__interruptable_foreach(void)
+{
+	int count = 0;
+	git_repository *repo = cl_git_sandbox_init("status");
+
+	cl_assert_equal_i(
+		GIT_EUSER, git_status_foreach(repo, cb_status__interrupt, &count)
+	);
+
+	cl_assert_equal_i(8, count);
+}