Make upload-pack report "not our ref" errors to the client. (If not, the
client would be left waiting for a response when the server is already
dead.)

Add tests to check the behavior of upload-pack and fetch-pack when
upload-pack is served from a changing repository (for example, when
different servers in a load-balancing agreement participate in the same
stateless RPC negotiation). This forms a baseline of comparison to the
ref-in-want functionality (which will be introduced in subsequent
commits), and ensures that subsequent commits do not change existing
behavior.

As part of this effort, a mechanism to substitute strings in an HTTP
response only on the first invocation is added.

Signed-off-by: Jonathan Tan <jonathanta...@google.com>
---
 t/lib-httpd.sh                     |  1 +
 t/lib-httpd/apache.conf            |  8 ++++
 t/lib-httpd/one-time-sed.sh        |  8 ++++
 t/t5552-upload-pack-ref-in-want.sh | 91 ++++++++++++++++++++++++++++++++++++++
 upload-pack.c                      |  6 ++-
 5 files changed, 113 insertions(+), 1 deletion(-)
 create mode 100644 t/lib-httpd/one-time-sed.sh

diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index 435a37465..84f8efdd4 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -132,6 +132,7 @@ prepare_httpd() {
        cp "$TEST_PATH"/passwd "$HTTPD_ROOT_PATH"
        install_script broken-smart-http.sh
        install_script error.sh
+       install_script one-time-sed.sh
 
        ln -s "$LIB_HTTPD_MODULE_PATH" "$HTTPD_ROOT_PATH/modules"
 
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 69174c6e3..ef218ff15 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -106,9 +106,14 @@ Alias /auth/dumb/ www/auth/dumb/
        SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
        SetEnv GIT_HTTP_EXPORT_ALL
 </LocationMatch>
+<LocationMatch /one_time_sed/>
+       SetEnv GIT_EXEC_PATH ${GIT_EXEC_PATH}
+       SetEnv GIT_HTTP_EXPORT_ALL
+</LocationMatch>
 ScriptAliasMatch /smart_*[^/]*/(.*) ${GIT_EXEC_PATH}/git-http-backend/$1
 ScriptAlias /broken_smart/ broken-smart-http.sh/
 ScriptAlias /error/ error.sh/
+ScriptAliasMatch /one_time_sed/(.*) one-time-sed.sh/$1
 <Directory ${GIT_EXEC_PATH}>
        Options FollowSymlinks
 </Directory>
@@ -118,6 +123,9 @@ ScriptAlias /error/ error.sh/
 <Files error.sh>
   Options ExecCGI
 </Files>
+<Files one-time-sed.sh>
+       Options ExecCGI
+</Files>
 <Files ${GIT_EXEC_PATH}/git-http-backend>
        Options ExecCGI
 </Files>
diff --git a/t/lib-httpd/one-time-sed.sh b/t/lib-httpd/one-time-sed.sh
new file mode 100644
index 000000000..060ec0300
--- /dev/null
+++ b/t/lib-httpd/one-time-sed.sh
@@ -0,0 +1,8 @@
+#!/bin/sh
+
+if [ -e one-time-sed ]; then
+       "$GIT_EXEC_PATH/git-http-backend" | sed "$(cat one-time-sed)"
+       rm one-time-sed
+else
+       "$GIT_EXEC_PATH/git-http-backend"
+fi
diff --git a/t/t5552-upload-pack-ref-in-want.sh 
b/t/t5552-upload-pack-ref-in-want.sh
index 3496af641..80cf2263a 100755
--- a/t/t5552-upload-pack-ref-in-want.sh
+++ b/t/t5552-upload-pack-ref-in-want.sh
@@ -292,4 +292,95 @@ test_expect_success 'hideRefs with namespaces' '
        check_output
 '
 
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+REPO="$HTTPD_DOCUMENT_ROOT_PATH/repo"
+LOCAL_PRISTINE="$(pwd)/local_pristine"
+
+test_expect_success 'setup repos for change-while-negotiating test' '
+       (
+               git init "$REPO" &&
+               cd "$REPO" &&
+               >.git/git-daemon-export-ok &&
+               test_commit m1 &&
+               git tag -d m1 &&
+
+               # Local repo with many commits (so that negotiation will take
+               # more than 1 request/response pair)
+               git clone "http://127.0.0.1:$LIB_HTTPD_PORT/smart/repo"; 
"$LOCAL_PRISTINE" &&
+               cd "$LOCAL_PRISTINE" &&
+               git checkout -b side &&
+               for i in $(seq 1 33); do test_commit s$i; done &&
+
+               # Add novel commits to upstream
+               git checkout master &&
+               cd "$REPO" &&
+               test_commit m2 &&
+               test_commit m3 &&
+               git tag -d m2 m3
+       ) &&
+       git -C "$LOCAL_PRISTINE" remote set-url origin 
"http://127.0.0.1:$LIB_HTTPD_PORT/one_time_sed/repo";
+'
+
+inconsistency() {
+       # Simulate that the server initially reports $2 as the ref
+       # corresponding to $1, and after that, $1 as the ref corresponding to
+       # $1. This corresponds to the real-life situation where the server's
+       # repository appears to change during negotiation, for example, when
+       # different servers in a load-balancing arrangement serve (stateless)
+       # RPCs during a single negotiation.
+       printf "s/%s/%s/" \
+              $(git -C "$REPO" rev-parse $1 | tr -d "\n") \
+              $(git -C "$REPO" rev-parse $2 | tr -d "\n") \
+              >"$HTTPD_ROOT_PATH/one-time-sed"
+}
+
+test_expect_success 'server is initially ahead - no ref in want' '
+       git -C "$REPO" config uploadpack.advertiseRefInWant false &&
+       rm -rf local &&
+       cp -r "$LOCAL_PRISTINE" local &&
+       inconsistency master 1234567890123456789012345678901234567890 &&
+       test_must_fail git -C local fetch 2> err &&
+       grep "ERR upload-pack: not our ref" err
+'
+
+test_expect_failure 'server is initially ahead - ref in want' '
+       git -C "$REPO" config uploadpack.advertiseRefInWant true &&
+       rm -rf local &&
+       cp -r "$LOCAL_PRISTINE" local &&
+       inconsistency master 1234567890123456789012345678901234567890 &&
+       git -C local fetch &&
+
+       git -C "$REPO" rev-parse --verify master > expected &&
+       git -C local rev-parse --verify refs/remotes/origin/master > actual &&
+       test_cmp expected actual
+'
+
+test_expect_success 'server is initially behind - no ref in want' '
+       git -C "$REPO" config uploadpack.advertiseRefInWant false &&
+       rm -rf local &&
+       cp -r "$LOCAL_PRISTINE" local &&
+       inconsistency master "master^" &&
+       git -C local fetch &&
+
+       git -C "$REPO" rev-parse --verify "master^" > expected &&
+       git -C local rev-parse --verify refs/remotes/origin/master > actual &&
+       test_cmp expected actual
+'
+
+test_expect_failure 'server is initially behind - ref in want' '
+       git -C "$REPO" config uploadpack.advertiseRefInWant true &&
+       rm -rf local &&
+       cp -r "$LOCAL_PRISTINE" local &&
+       inconsistency master "master^" &&
+       git -C local fetch &&
+
+       git -C "$REPO" rev-parse --verify "master" > expected &&
+       git -C local rev-parse --verify refs/remotes/origin/master > actual &&
+       test_cmp expected actual
+'
+
+stop_httpd
+
 test_done
diff --git a/upload-pack.c b/upload-pack.c
index b88ed8e26..0678c53d6 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -862,9 +862,13 @@ static void receive_needs(struct string_list 
*wanted_ns_refs)
                } else if (skip_prefix(line, "want ", &arg) &&
                           !get_sha1_hex(arg, sha1_buf)) {
                        o = parse_object(sha1_buf);
-                       if (!o)
+                       if (!o) {
+                               packet_write_fmt(1,
+                                                "ERR upload-pack: not our ref 
%s",
+                                                sha1_to_hex(sha1_buf));
                                die("git upload-pack: not our ref %s",
                                    sha1_to_hex(sha1_buf));
+                       }
                        if (!(o->flags & WANTED)) {
                                o->flags |= WANTED;
                                if (!((allow_unadvertised_object_request & 
ALLOW_ANY_SHA1) == ALLOW_ANY_SHA1
-- 
2.11.0.483.g087da7b7c-goog

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