is_refname_available(): revamp the comments

Change the comments to a running example of running the function with
refname set to "refs/foo/bar". Add some more explanation of the logic.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

diff --git a/refs.c b/refs.c
index 47e4e53..776bbce 100644 (file)
--- a/refs.c
+++ b/refs.c
@@ -876,9 +876,9 @@ static void report_refname_conflict(struct ref_entry *entry,
  * operation).
  *
  * Two reference names conflict if one of them exactly matches the
- * leading components of the other; e.g., "foo/bar" conflicts with
- * both "foo" and with "foo/bar/baz" but not with "foo/bar" or
- * "foo/barbados".
+ * leading components of the other; e.g., "refs/foo/bar" conflicts
+ * with both "refs/foo" and with "refs/foo/bar/baz" but not with
+ * "refs/foo/bar" or "refs/foo/barbados".
  *
  * skip must be sorted.
  */
@@ -891,19 +891,39 @@ static int is_refname_available(const char *refname,
        int pos;
        char *dirname;
 
+       /*
+        * For the sake of comments in this function, suppose that
+        * refname is "refs/foo/bar".
+        */
+
        for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
                /*
-                * We are still at a leading dir of the refname; we are
-                * looking for a conflict with a leaf entry.
-                *
-                * If we find one, we still must make sure it is
-                * not in "skip".
+                * We are still at a leading dir of the refname (e.g.,
+                * "refs/foo"; if there is a reference with that name,
+                * it is a conflict, *unless* it is in skip.
                 */
                pos = search_ref_dir(dir, refname, slash - refname);
                if (pos >= 0) {
+                       /*
+                        * We found a reference whose name is a proper
+                        * prefix of refname; e.g., "refs/foo".
+                        */
                        struct ref_entry *entry = dir->entries[pos];
-                       if (entry_matches(entry, skip))
+                       if (entry_matches(entry, skip)) {
+                               /*
+                                * The reference we just found, e.g.,
+                                * "refs/foo", is also in skip, so it
+                                * is not considered a conflict.
+                                * Moreover, the fact that "refs/foo"
+                                * exists means that there cannot be
+                                * any references anywhere under the
+                                * "refs/foo/" namespace (because they
+                                * would have conflicted with
+                                * "refs/foo"). So we can stop looking
+                                * now and return true.
+                                */
                                return 1;
+                       }
                        report_refname_conflict(entry, refname);
                        return 0;
                }
@@ -911,19 +931,29 @@ static int is_refname_available(const char *refname,
 
                /*
                 * Otherwise, we can try to continue our search with
-                * the next component; if we come up empty, we know
-                * there is nothing under this whole prefix.
+                * the next component. So try to look up the
+                * directory, e.g., "refs/foo/".
                 */
                pos = search_ref_dir(dir, refname, slash + 1 - refname);
-               if (pos < 0)
+               if (pos < 0) {
+                       /*
+                        * There was no directory "refs/foo/", so
+                        * there is nothing under this whole prefix,
+                        * and we are OK.
+                        */
                        return 1;
+               }
 
                dir = get_ref_dir(dir->entries[pos]);
        }
 
        /*
-        * We are at the leaf of our refname; we want to
-        * make sure there are no directories which match it.
+        * We are at the leaf of our refname (e.g., "refs/foo/bar").
+        * There is no point in searching for a reference with that
+        * name, because a refname isn't considered to conflict with
+        * itself. But we still need to check for references whose
+        * names are in the "refs/foo/bar/" namespace, because they
+        * *do* conflict.
         */
        len = strlen(refname);
        dirname = xmallocz(len + 1);
@@ -933,9 +963,9 @@ static int is_refname_available(const char *refname,
 
        if (pos >= 0) {
                /*
-                * We found a directory named "refname". It is a
-                * problem iff it contains any ref that is not
-                * in "skip".
+                * We found a directory named "$refname/" (e.g.,
+                * "refs/foo/bar/"). It is a problem iff it contains
+                * any ref that is not in "skip".
                 */
                struct ref_entry *entry = dir->entries[pos];
                struct ref_dir *dir = get_ref_dir(entry);
@@ -950,11 +980,6 @@ static int is_refname_available(const char *refname,
                return 0;
        }
 
-       /*
-        * There is no point in searching for another leaf
-        * node which matches it; such an entry would be the
-        * ref we are looking for, not a conflict.
-        */
        return 1;
 }