Added a GIT_OID_MINPREFIXLEN constant to define the minimum length allowed for…

Added a GIT_OID_MINPREFIXLEN constant to define the minimum length allowed for oid prefixes (set to 4, like in git). Consequently updated some object lookup methods and their documentation.

include/git2/object.h

@@ -65,9 +65,9 @@ GIT_EXTERN(int) git_object_lookup(git_object **object, git_repository *repo, con
  * The object obtained will be so that its identifier
  * matches the first 'len' hexadecimal characters
  * (packets of 4 bits) of the given 'id'.
- * 'len' must be long enough to identify a unique
- * object matching the prefix; otherwise the method will
- * fail.
+ * 'len' must be at least GIT_OID_MINPREFIXLEN, and
+ * long enough to identify a unique object matching
+ * the prefix; otherwise the method will fail.
  *
  * The generated reference is owned by the repository and
  * should be closed with the `git_object_close` method

include/git2/odb.h

@@ -132,9 +132,10 @@ GIT_EXTERN(int) git_odb_read(git_odb_object **out, git_odb *db, const git_oid *i
  * This method queries all available ODB backends
  * trying to match the 'len' first hexadecimal
  * characters of the 'short_id'.
- * The remaining bits (GIT_OID_HEXSZ-len)*4 bits of
+ * The remaining (GIT_OID_HEXSZ-len)*4 bits of
  * 'short_id' must be 0s.
- * The prefix must be long enough to identify
+ * 'len' must be at least GIT_OID_MINPREFIXLEN,
+ * and the prefix must be long enough to identify
  * a unique object in all the backends; the
  * method will fail otherwise.
  *

include/git2/oid.h

@@ -43,6 +43,10 @@ GIT_BEGIN_DECL
 /** Size (in bytes) of a hex formatted oid */
 #define GIT_OID_HEXSZ (GIT_OID_RAWSZ * 2)
 
+/** Minimum length (in number of hex characters,
+ * i.e. packets of 4 bits) of an oid prefix */
+#define GIT_OID_MINPREFIXLEN 4
+
 /** Unique identity of any object (commit, tree, blob, tag). */
 typedef struct {
 	/** raw binary formatted id */

src/object.c

@@ -104,8 +104,8 @@ int git_object_lookup_short_oid(git_object **object_out, git_repository *repo, c
 
 	assert(repo && object_out && id);
 
-	if (len == 0)
-		return git__throw(GIT_EAMBIGUOUSOIDPREFIX, "Failed to lookup object. Prefix length should be not be 0.");
+	if (len < GIT_OID_MINPREFIXLEN)
+		return git__throw(GIT_EAMBIGUOUSOIDPREFIX, "Failed to lookup object. Prefix length is lower than %d.", GIT_OID_MINPREFIXLEN);
 	if (len > GIT_OID_HEXSZ) {
 		len = GIT_OID_HEXSZ;
 	}

src/odb_pack.c

@@ -268,6 +268,8 @@ static off_t nth_packed_object_offset(const struct pack_file *p, uint32_t n);
  * a prefix of an identifier.
  * Throws GIT_EAMBIGUOUSOIDPREFIX if short oid
  * is ambiguous within the pack.
+ * This method assumes that len is between
+ * GIT_OID_MINPREFIXLEN and GIT_OID_HEXSZ.
  */
 static int pack_entry_find_offset(
 		off_t *offset_out,
@@ -289,6 +291,8 @@ static int pack_entry_find(struct pack_entry *e,
  * a prefix of an identifier.
  * Throws GIT_EAMBIGUOUSOIDPREFIX if short oid
  * is ambiguous.
+ * This method assumes that len is between
+ * GIT_OID_MINPREFIXLEN and GIT_OID_HEXSZ.
  */
 static int pack_entry_find_unique_short_oid(struct pack_entry *e,
 					struct pack_backend *backend,