[PATCH v4 6/6] get_short_oid: document & warn if we ignore the type selector

[Ævar Arnfjörð Bjarmason]

The SHA1 prefix 06fa currently matches no blobs in git.git. When
disambiguating short SHA1s we've been quietly ignoring the user's type
selector as a fallback mechanism, this was intentionally added in
1ffa26c461 ("get_short_sha1: list ambiguous objects on error",
2016-09-26).

I think that behavior makes sense, it's not very useful to just show
nothing because a preference has been expressed via core.disambiguate,
but it's bad that we're quietly doing this. The user might thing that
we just didn't understand what e.g 06fa^{blob} meant.

Now we'll instead print a warning if no objects of the requested type
were found:

    $ git rev-parse 06fa^{blob}
    error: short SHA1 06fa is ambiguous
    hint: The candidates are:
    [... no blobs listed ...]
    warning: Your hint (via core.disambiguate or peel syntax) was ignored, we 
fell
    back to showing all object types since no object of the requested type
    matched the provide short SHA1 06fa

Signed-off-by: Ævar Arnfjörð Bjarmason <ava...@gmail.com>
---
 sha1-name.c                         | 11 ++++++++++-
 t/t1512-rev-parse-disambiguation.sh |  5 ++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/sha1-name.c b/sha1-name.c
index 46d8b1afa6..df33cc2dba 100644
--- a/sha1-name.c
+++ b/sha1-name.c
@@ -438,6 +438,7 @@ static int get_short_oid(const char *name, int len, struct 
object_id *oid,
 
        if (!quietly && (status == SHORT_NAME_AMBIGUOUS)) {
                struct oid_array collect = OID_ARRAY_INIT;
+               int ignored_hint = 0;
 
                error(_("short SHA1 %s is ambiguous"), ds.hex_pfx);
 
@@ -447,8 +448,10 @@ static int get_short_oid(const char *name, int len, struct 
object_id *oid,
                 * that case, we still want to show them, so disable the hint
                 * function entirely.
                 */
-               if (!ds.ambiguous)
+               if (!ds.ambiguous) {
                        ds.fn = NULL;
+                       ignored_hint = 1;
+               }
 
                advise(_("The candidates are:"));
                for_each_abbrev(ds.hex_pfx, collect_ambiguous, &collect);
@@ -457,6 +460,12 @@ static int get_short_oid(const char *name, int len, struct 
object_id *oid,
                if (oid_array_for_each(&collect, show_ambiguous_object, &ds))
                        BUG("show_ambiguous_object shouldn't return non-zero");
                oid_array_clear(&collect);
+
+               if (ignored_hint) {
+                       warning(_("Your hint (via core.disambiguate or peel 
syntax) was ignored, we fell\n"
+                                 "back to showing all object types since no 
object of the requested type\n"
+                                 "matched the provide short SHA1 %s"), 
ds.hex_pfx);
+               }
        }
 
        return status;
diff --git a/t/t1512-rev-parse-disambiguation.sh 
b/t/t1512-rev-parse-disambiguation.sh
index 2701462041..1f06c1e61f 100755
--- a/t/t1512-rev-parse-disambiguation.sh
+++ b/t/t1512-rev-parse-disambiguation.sh
@@ -344,7 +344,10 @@ test_expect_success C_LOCALE_OUTPUT 'failed type-selector 
still shows hint' '
        echo 872 | git hash-object --stdin -w &&
        test_must_fail git rev-parse ee3d^{commit} 2>stderr &&
        grep ^hint: stderr >hints &&
-       test_line_count = 3 hints
+       test_line_count = 3 hints &&
+       grep ^warning stderr >warnings &&
+       grep -q "Your hint.*was ignored" warnings &&
+       grep -q "the provide short SHA1 ee3d" stderr
 '
 
 test_expect_success 'core.disambiguate config can prefer types' '
-- 
2.17.0.410.g4ac3413cc8