odb: remove mutual recursion when parsing alternates

When adding an alternative object database source we not only have to
consider the added source itself, but we also have to add _its_ sources
to our database. We implement this via mutual recursion:

  1. We first call `link_alt_odb_entries()`.

  2. `link_alt_odb_entries()` calls `parse_alternates()`.

  3. We then add each alternate via `odb_add_alternate_recursively()`.

  4. `odb_add_alternate_recursively()` calls `link_alt_odb_entries()`
     again.

This flow is somewhat hard to follow, but more importantly it means that
parsing of alternates is somewhat tied to the recursive behaviour.

Refactor the function to remove the mutual recursion between adding
sources and parsing alternates. The parsing step thus becomes completely
oblivious to the fact that there is recursive behaviour going on at all.
The recursion is handled by `odb_add_alternate_recursively()` instead,
which now recurses with itself.

This refactoring allows us to move parsing of alternates into object
database sources in a subsequent step.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/odb.c b/odb.c
index 3112eab5d03ed1d7caee51faf85e4254415c54c8..59944d46496e9c878499b9c39d5b6d0400c40e2e 100644 (file)
--- a/odb.c
+++ b/odb.c
@@ -147,9 +147,8 @@ static bool odb_is_source_usable(struct object_database *o, const char *path)
  * of the object ID, an extra slash for the first level indirection, and
  * the terminating NUL.
  */
-static void read_info_alternates(struct object_database *odb,
-                                const char *relative_base,
-                                int depth);
+static void read_info_alternates(const char *relative_base,
+                                struct strvec *out);
 
 static struct odb_source *odb_source_new(struct object_database *odb,
                                         const char *path,
@@ -171,6 +170,7 @@ static struct odb_source *odb_add_alternate_recursively(struct object_database *
                                                        int depth)
 {
        struct odb_source *alternate = NULL;
+       struct strvec sources = STRVEC_INIT;
        khiter_t pos;
        int ret;
 
@@ -189,9 +189,17 @@ static struct odb_source *odb_add_alternate_recursively(struct object_database *
        kh_value(odb->source_by_path, pos) = alternate;
 
        /* recursively add alternates */
-       read_info_alternates(odb, alternate->path, depth + 1);
+       read_info_alternates(alternate->path, &sources);
+       if (sources.nr && depth + 1 > 5) {
+               error(_("%s: ignoring alternate object stores, nesting too deep"),
+                     source);
+       } else {
+               for (size_t i = 0; i < sources.nr; i++)
+                       odb_add_alternate_recursively(odb, sources.v[i], depth + 1);
+       }
 
  error:
+       strvec_clear(&sources);
        return alternate;
 }
 
@@ -203,6 +211,9 @@ static void parse_alternates(const char *string,
        struct strbuf pathbuf = STRBUF_INIT;
        struct strbuf buf = STRBUF_INIT;
 
+       if (!string || !*string)
+               return;
+
        while (*string) {
                const char *end;
 
@@ -259,34 +270,11 @@ static void parse_alternates(const char *string,
        strbuf_release(&buf);
 }
 
-static void link_alt_odb_entries(struct object_database *odb, const char *alt,
-                                int sep, const char *relative_base, int depth)
+static void read_info_alternates(const char *relative_base,
+                                struct strvec *out)
 {
-       struct strvec alternates = STRVEC_INIT;
-
-       if (!alt || !*alt)
-               return;
-
-       if (depth > 5) {
-               error(_("%s: ignoring alternate object stores, nesting too deep"),
-                               relative_base);
-               return;
-       }
-
-       parse_alternates(alt, sep, relative_base, &alternates);
-
-       for (size_t i = 0; i < alternates.nr; i++)
-               odb_add_alternate_recursively(odb, alternates.v[i], depth);
-
-       strvec_clear(&alternates);
-}
-
-static void read_info_alternates(struct object_database *odb,
-                                const char *relative_base,
-                                int depth)
-{
-       char *path;
        struct strbuf buf = STRBUF_INIT;
+       char *path;
 
        path = xstrfmt("%s/info/alternates", relative_base);
        if (strbuf_read_file(&buf, path, 1024) < 0) {
@@ -294,8 +282,8 @@ static void read_info_alternates(struct object_database *odb,
                free(path);
                return;
        }
+       parse_alternates(buf.buf, '\n', relative_base, out);
 
-       link_alt_odb_entries(odb, buf.buf, '\n', relative_base, depth);
        strbuf_release(&buf);
        free(path);
 }
@@ -622,13 +610,19 @@ int odb_for_each_alternate(struct object_database *odb,
 
 void odb_prepare_alternates(struct object_database *odb)
 {
+       struct strvec sources = STRVEC_INIT;
+
        if (odb->loaded_alternates)
                return;
 
-       link_alt_odb_entries(odb, odb->alternate_db, PATH_SEP, NULL, 0);
+       parse_alternates(odb->alternate_db, PATH_SEP, NULL, &sources);
+       read_info_alternates(odb->sources->path, &sources);
+       for (size_t i = 0; i < sources.nr; i++)
+               odb_add_alternate_recursively(odb, sources.v[i], 0);
 
-       read_info_alternates(odb, odb->sources->path, 0);
        odb->loaded_alternates = 1;
+
+       strvec_clear(&sources);
 }
 
 int odb_has_alternates(struct object_database *odb)