refs: Speed up packed lookups.

Currently ref lookups require loading the entire packed-refs file into
a hashmap in memory. For repos with large numbers of refs this can be
painfully slow.

This patch replaces the existing lookup code and instead mmap()'s the
packed-refs file and performs a binary search to locate the ref entry.
Git uses a similiar approach.

The old hash table codepath is still used for unsorted packed-refs files.

This patch also fixes a minor bug where the "peeled" trait is never
parsed correctly from the packed-refs header.

docs/changelog.md

@@ -1963,3 +1963,8 @@ v0.22
   functions. This is not something which we can know to do. A
   last-resort convenience function is provided in sys/openssl.h,
   `git_openssl_set_locking()` which can be used to set the locking.
+
+* `git_reference_*()` functions use mmap() + binary search for packed
+  refs lookups when using the fs backend. Previously all entries were
+  read into a hashtable, which could be slow for repositories with a
+  large number of refs.

src/refdb_fs.c

@@ -65,9 +65,13 @@ typedef struct refdb_fs_backend {
 	git_iterator_flag_t iterator_flags;
 	uint32_t direach_flags;
 	int fsync;
+	git_map packed_refs_map;
+	git_mutex prlock; /* protect packed_refs_map */
+	bool sorted;
 } refdb_fs_backend;
 
 static int refdb_reflog_fs__delete(git_refdb_backend *_backend, const char *name);
+static const char * packed_set_peeling_mode(const char *data, size_t data_sz, refdb_fs_backend *backend);
 
 GIT_INLINE(int) loose_path(
 	git_str *out,
@@ -137,27 +141,9 @@ static int packed_reload(refdb_fs_backend *backend)
 	scan = (char *)packedrefs.ptr;
 	eof  = scan + packedrefs.size;
 
-	backend->peeling_mode = PEELING_NONE;
-
-	if (*scan == '#') {
-		static const char *traits_header = "# pack-refs with: ";
-
-		if (git__prefixcmp(scan, traits_header) == 0) {
-			scan += strlen(traits_header);
-			eol = strchr(scan, '\n');
-
-			if (!eol)
-				goto parse_failed;
-			*eol = '\0';
-
-			if (strstr(scan, " fully-peeled ") != NULL) {
-				backend->peeling_mode = PEELING_FULL;
-			} else if (strstr(scan, " peeled ") != NULL) {
-				backend->peeling_mode = PEELING_STANDARD;
-			}
-
-			scan = eol + 1;
-		}
+	scan = (char *)packed_set_peeling_mode(scan, packedrefs.size, backend);
+	if (!scan) {
+		goto parse_failed;
 	}
 
 	while (scan < eof && *scan == '#') {
@@ -466,10 +452,176 @@ static int ref_error_notfound(const char *name)
 	return GIT_ENOTFOUND;
 }
 
-static int packed_lookup(
-	git_reference **out,
-	refdb_fs_backend *backend,
-	const char *ref_name)
+static const char *packed_set_peeling_mode(
+        const char *data,
+        size_t data_sz,
+        refdb_fs_backend *backend)
+{
+	static const char *traits_header = "# pack-refs with:";
+	const char *eol;
+	backend->peeling_mode = PEELING_NONE;
+
+	if (data_sz == 0 || *data != '#') {
+		return data;
+	}
+
+	if (git__prefixncmp(data, data_sz, traits_header) == 0) {
+		size_t hdr_sz = strlen(traits_header);
+		const char *sorted = " sorted ";
+		const char *peeled = " peeled ";
+		const char *fully_peeled = " fully-peeled ";
+		data += hdr_sz;
+		data_sz -= hdr_sz;
+
+		eol = memchr(data, '\n', data_sz);
+
+		if (!eol)
+			return NULL;
+
+		if (git__memmem(data, eol - data, fully_peeled, strlen(fully_peeled))) {
+			backend->peeling_mode = PEELING_FULL;
+		} else if (git__memmem(data, eol - data, peeled, strlen(peeled))) {
+			backend->peeling_mode = PEELING_STANDARD;
+		}
+
+		if (git__memmem(data, eol - data, sorted, strlen(sorted))) {
+			backend->sorted = true;
+		} else {
+			backend->sorted = false;
+		}
+
+		return eol + 1;
+	}
+	return data;
+}
+
+static int packed_map_check(refdb_fs_backend *backend)
+{
+	int error = 0;
+	git_file fd = -1;
+	struct stat st;
+
+	if ((error = git_mutex_lock(&backend->prlock)) < 0) {
+		return error;
+	}
+
+	if (backend->packed_refs_map.data) {
+		git_mutex_unlock(&backend->prlock);
+		return error;
+	}
+
+	fd = git_futils_open_ro(backend->refcache->path);
+	if (fd < 0) {
+		git_mutex_unlock(&backend->prlock);
+		if (fd == GIT_ENOTFOUND) {
+			git_error_clear();
+			return 0;
+		}
+		return fd;
+	}
+
+	if (p_fstat(fd, &st) < 0) {
+		p_close(fd);
+		git_mutex_unlock(&backend->prlock);
+		git_error_set(GIT_ERROR_OS, "unable to stat packed-refs '%s'", backend->refcache->path);
+		return -1;
+	}
+
+	if (st.st_size == 0) {
+		p_close(fd);
+		git_mutex_unlock(&backend->prlock);
+		return 0;
+	}
+
+	error = git_futils_mmap_ro(&backend->packed_refs_map, fd, 0, (size_t)st.st_size);
+	p_close(fd);
+	if (error < 0) {
+		git_mutex_unlock(&backend->prlock);
+		return error;
+	}
+
+	packed_set_peeling_mode(
+	        backend->packed_refs_map.data, backend->packed_refs_map.len,
+	        backend);
+
+	git_mutex_unlock(&backend->prlock);
+	return error;
+}
+
+/*
+ * Find beginning of packed-ref record pointed to by p.
+ *   buf - a lower-bound pointer to some memory buffer
+ *   p - an upper-bound pointer to the same memory buffer
+ */
+static const char *start_of_record(const char *buf, const char *p)
+{
+	const char *nl = p;
+	while (1) {
+		nl = git__memrchr(buf, '\n', nl - buf);
+		if (!nl)
+			return buf;
+
+		if (nl[1] == '^' && nl > buf)
+			--nl;
+		else
+			break;
+	};
+	return nl + 1;
+}
+
+/*
+ * Find end of packed-ref record pointed to by p.
+ *   end - an upper-bound pointer to some memory buffer
+ *   p - a lower-bound pointer to the same memory buffer
+ */
+static const char *end_of_record(const char *p, const char *end)
+{
+	while (1) {
+		size_t sz = end - p;
+		p = memchr(p, '\n', sz);
+		if (!p) {
+			return end;
+		}
+		++p;
+		if (p < end && p[0] == '^')
+			++p;
+		else
+			break;
+	}
+	return p;
+}
+
+static int
+cmp_record_to_refname(const char *rec, size_t data_end, const char *ref_name)
+{
+	const size_t ref_len = strlen(ref_name);
+	int cmp_val;
+	const char *end;
+
+	rec += GIT_OID_HEXSZ + 1; /* <oid> + space */
+	if (data_end < GIT_OID_HEXSZ + 3) {
+		/* an incomplete (corrupt) record is treated as less than ref_name */
+		return -1;
+	}
+	data_end -= GIT_OID_HEXSZ + 1;
+
+	end = memchr(rec, '\n', data_end);
+	if (end) {
+		data_end = end - rec;
+	}
+
+	cmp_val = memcmp(rec, ref_name, min(ref_len, data_end));
+
+	if (cmp_val == 0 && data_end != ref_len) {
+		return (data_end > ref_len) ? 1 : -1;
+	}
+	return cmp_val;
+}
+
+static int packed_unsorted_lookup(
+        git_reference **out,
+        refdb_fs_backend *backend,
+        const char *ref_name)
 {
 	int error = 0;
 	struct packref *entry;
@@ -494,6 +646,86 @@ static int packed_lookup(
 	return error;
 }
 
+static int packed_lookup(
+        git_reference **out,
+        refdb_fs_backend *backend,
+        const char *ref_name)
+{
+	int error = 0;
+	const char *left, *right, *data_end;
+
+	if ((error = packed_map_check(backend)) < 0)
+		return error;
+
+	if (!backend->sorted) {
+		return packed_unsorted_lookup(out, backend, ref_name);
+	}
+
+	left = backend->packed_refs_map.data;
+	right = data_end = ((const char *)backend->packed_refs_map.data) +
+	                   backend->packed_refs_map.len;
+
+	while (left < right && *left == '#') {
+		if (!(left = memchr(left, '\n', data_end - left)))
+			goto parse_failed;
+		left++;
+	}
+
+	while (left < right) {
+		const char *mid, *rec;
+		int compare;
+
+		mid = left + (right - left) / 2;
+		rec = start_of_record(left, mid);
+		compare = cmp_record_to_refname(rec, data_end - rec, ref_name);
+
+		if (compare < 0) {
+			left = end_of_record(mid, right);
+		} else if (compare > 0) {
+			right = rec;
+		} else {
+			const char *eol;
+			git_oid oid, peel, *peel_ptr = NULL;
+
+			if (data_end - rec < GIT_OID_HEXSZ ||
+			    git_oid_fromstr(&oid, rec) < 0) {
+				goto parse_failed;
+			}
+			rec += GIT_OID_HEXSZ + 1;
+			if (!(eol = memchr(rec, '\n', data_end - rec))) {
+				goto parse_failed;
+			}
+
+			/* look for optional "^<OID>\n" */
+
+			if (eol + 1 < data_end) {
+				rec = eol + 1;
+
+				if (*rec == '^') {
+					rec++;
+					if (data_end - rec < GIT_OID_HEXSZ ||
+						git_oid_fromstr(&peel, rec) < 0) {
+						goto parse_failed;
+					}
+					peel_ptr = &peel;
+				}
+			}
+
+			*out = git_reference__alloc(ref_name, &oid, peel_ptr);
+			if (!*out) {
+				return -1;
+			}
+
+			return 0;
+		}
+	}
+	return GIT_ENOTFOUND;
+
+parse_failed:
+	git_error_set(GIT_ERROR_REFERENCE, "corrupted packed references file");
+	return -1;
+}
+
 static int refdb_fs_backend__lookup(
 	git_reference **out,
 	git_refdb_backend *_backend,
@@ -513,7 +745,6 @@ static int refdb_fs_backend__lookup(
 		git_error_clear();
 		error = packed_lookup(out, backend, ref_name);
 	}
-
 	return error;
 }
 
@@ -1081,6 +1312,18 @@ static int packed_write(refdb_fs_backend *backend)
 	int error, open_flags = 0;
 	size_t i;
 
+	/* take lock and close up packed-refs mmap if open */
+	if ((error = git_mutex_lock(&backend->prlock)) < 0) {
+		return error;
+	}
+
+	if (backend->packed_refs_map.data) {
+		git_futils_mmap_free(&backend->packed_refs_map);
+		backend->packed_refs_map.data = NULL;
+	}
+
+	git_mutex_unlock(&backend->prlock);
+
 	/* lock the cache to updates while we do this */
 	if ((error = git_sortedcache_wlock(refcache)) < 0)
 		return error;
@@ -1568,6 +1811,15 @@ static void refdb_fs_backend__free(git_refdb_backend *_backend)
 		return;
 
 	git_sortedcache_free(backend->refcache);
+
+	git_mutex_lock(&backend->prlock);
+	if (backend->packed_refs_map.data) {
+		git_futils_mmap_free(&backend->packed_refs_map);
+		backend->packed_refs_map.data = NULL;
+	}
+	git_mutex_unlock(&backend->prlock);
+	git_mutex_free(&backend->prlock);
+
 	git__free(backend->gitpath);
 	git__free(backend->commonpath);
 	git__free(backend);
@@ -2121,6 +2373,11 @@ int git_refdb_backend_fs(
 
 	backend = git__calloc(1, sizeof(refdb_fs_backend));
 	GIT_ERROR_CHECK_ALLOC(backend);
+	if (git_mutex_init(&backend->prlock) < 0) {
+		git__free(backend);
+		return -1;
+	}
+
 
 	if (git_refdb_init_backend(&backend->parent, GIT_REFDB_BACKEND_VERSION) < 0)
 		goto fail;
@@ -2183,6 +2440,7 @@ int git_refdb_backend_fs(
 	return 0;
 
 fail:
+	git_mutex_free(&backend->prlock);
 	git_str_dispose(&gitpath);
 	git__free(backend->gitpath);
 	git__free(backend->commonpath);

tests/refs/crashes.c

 #include "clar_libgit2.h"
+#include "refs.h"
 
 void test_refs_crashes__double_free(void)
 {
@@ -18,3 +19,26 @@ void test_refs_crashes__double_free(void)
 
 	cl_git_sandbox_cleanup();
 }
+
+void test_refs_crashes__empty_packedrefs(void)
+{
+	git_repository *repo;
+	git_reference *ref;
+	const char *REFNAME = "refs/heads/xxx";
+	git_str temp_path = GIT_STR_INIT;
+	int fd = 0;
+
+	repo = cl_git_sandbox_init("empty_bare.git");
+
+	/* create zero-length packed-refs file */
+	cl_git_pass(git_str_joinpath(&temp_path, git_repository_path(repo), GIT_PACKEDREFS_FILE));
+	cl_git_pass(((fd = p_creat(temp_path.ptr, 0644)) < 0));
+	cl_git_pass(p_close(fd));
+
+	/* should fail gracefully */
+	cl_git_fail_with(
+	        GIT_ENOTFOUND, git_reference_lookup(&ref, repo, REFNAME));
+
+	cl_git_sandbox_cleanup();
+	git_str_dispose(&temp_path);
+}

tests/resources/peeled.git/packed-refs

-# pack-refs with: peeled fully-peeled 
+# pack-refs with: peeled fully-peeled sorted 
 c2596aa0151888587ec5c0187f261e63412d9e11 refs/foo/tag-outside-tags
 ^0df1a5865c8abfc09f1f2182e6a31be550e99f07
 0df1a5865c8abfc09f1f2182e6a31be550e99f07 refs/heads/master

tests/resources/testrepo.git/packed-refs

-# pack-refs with: peeled 
+# pack-refs with: peeled sorted 
 41bc8c69075bbdb46c5c6f0566cc8cc5b46e8bd9 refs/heads/packed
 5b5b025afb0b4c913b4c338a42934a3863bf3644 refs/heads/packed-test

tests/resources/testrepo/.gitted/packed-refs

-# pack-refs with: peeled 
+# pack-refs with: peeled sorted 
 41bc8c69075bbdb46c5c6f0566cc8cc5b46e8bd9 refs/heads/packed
 5b5b025afb0b4c913b4c338a42934a3863bf3644 refs/heads/packed-test
 b25fa35b38051e4ae45d4222e795f9df2e43f1d1 refs/tags/packed-tag

tests/resources/testrepo2/.gitted/packed-refs

-# pack-refs with: peeled fully-peeled 
+# pack-refs with: peeled fully-peeled sorted 
 36060c58702ed4c2a40832c51758d5344201d89a refs/remotes/origin/master
 41bc8c69075bbdb46c5c6f0566cc8cc5b46e8bd9 refs/remotes/origin/packed
 5b5b025afb0b4c913b4c338a42934a3863bf3644 refs/tags/v0.9