Re: [RFC 4/4] Add cat-blob report pipe from fast-import to remote-helper.

On invocation of a helper a new pipe is opened.
To close the other end after fork, the prexec_cb feature
of the run_command api is used.
If the helper is not used with fast-import later the pipe
is unused.
The FD is passed to the remote-helper via it's environment,
helpers that don't use fast-import can simply ignore it.
fast-import has an argv for that.

Signed-off-by: Florian Achleitner <redacted>
---
 transport-helper.c |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 vcs-svn/svndump.c  |    9 ++++++++-
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/transport-helper.c b/transport-helper.c
index 61c928f..b438040 100644
--- a/transport-helper.c
+++ b/transport-helper.c

@@ -17,6 +17,7 @@ struct helper_data {

       const char *name;
       struct child_process *helper;
       FILE *out;
+       int fast_import_backchannel_pipe[2];
       unsigned fetch : 1,
               import : 1,
               export : 1,

@@ -98,19 +99,30 @@ static void do_take_over(struct transport *transport)

       free(data);
 }

+static int fd_to_close;
+void close_fd_prexec_cb(void)
+{
+       if(debug)
+               fprintf(stderr, "close_fd_prexec_cb closing %d\n", fd_to_close);
+       close(fd_to_close);
+}
+
 static struct child_process *get_helper(struct transport *transport)
 {
       struct helper_data *data = transport->data;
       struct strbuf buf = STRBUF_INIT;
+       struct strbuf envbuf = STRBUF_INIT;
       struct child_process *helper;
       const char **refspecs = NULL;
       int refspec_nr = 0;
       int refspec_alloc = 0;
       int duped;
       int code;
+       int err;
       char git_dir_buf[sizeof(GIT_DIR_ENVIRONMENT) + PATH_MAX + 1];
       const char *helper_env[] = {
               git_dir_buf,
+               NULL,   /* placeholder */
               NULL
       };

@@ -133,6 +145,24 @@ static struct child_process *get_helper(struct transport *transport)

       snprintf(git_dir_buf, sizeof(git_dir_buf), "%s=%s", GIT_DIR_ENVIRONMENT, get_git_dir());
       helper->env = helper_env;

+
+       /* create an additional pipe from fast-import to the helper */
+       err = pipe(data->fast_import_backchannel_pipe);
+       if(err)
+               die("cannot create fast_import_backchannel_pipe: %s", strerror(errno));
+
+       if(debug)
+               fprintf(stderr, "Remote helper created fast_import_backchannel_pipe { %d, %d }\n",
+                               data->fast_import_backchannel_pipe[0], data->fast_import_backchannel_pipe[1]);
+
+       strbuf_addf(&envbuf, "GIT_REPORT_FILENO=%d", data->fast_import_backchannel_pipe[0]);
+       helper_env[1] = strbuf_detach(&envbuf, NULL);
+
+       /* after the fork, we need to close the write end in the helper */
+       fd_to_close = data->fast_import_backchannel_pipe[1];
+       /* the prexec callback is run just before exec */
+       helper->preexec_cb = close_fd_prexec_cb;
+
       code = start_command(helper);
       if (code < 0 && errno == ENOENT)
               die("Unable to find remote helper for '%s'", data->name);

@@ -141,6 +171,7 @@ static struct child_process *get_helper(struct transport *transport)

       data->helper = helper;
       data->no_disconnect_req = 0;
+       free((void*)helper_env[1]);

       /*
        * Open the output as FILE* so strbuf_getline() can be used.

@@ -237,6 +268,10 @@ static int disconnect_helper(struct transport *transport)

                       xwrite(data->helper->in, "\n", 1);
                       sigchain_pop(SIGPIPE);
               }
+               /* close the pipe, it is still open if it wasn't used for fast-import. */
+               close(data->fast_import_backchannel_pipe[0]);
+               close(data->fast_import_backchannel_pipe[1]);
+
               close(data->helper->in);
               close(data->helper->out);
               fclose(data->out);

@@ -376,13 +411,20 @@ static int fetch_with_fetch(struct transport *transport,

 static int get_importer(struct transport *transport, struct child_process *fastimport)
 {
       struct child_process *helper = get_helper(transport);
+       struct helper_data *data = transport->data;
+       struct strbuf buf = STRBUF_INIT;
       memset(fastimport, 0, sizeof(*fastimport));
       fastimport->in = helper->out;
       fastimport->argv = xcalloc(5, sizeof(*fastimport->argv));
       fastimport->argv[0] = "fast-import";
       fastimport->argv[1] = "--quiet";
+       strbuf_addf(&buf, "--cat-blob-fd=%d", data->fast_import_backchannel_pipe[1]);
+       fastimport->argv[2] = strbuf_detach(&buf, NULL);

       fastimport->git_cmd = 1;
+
+       fd_to_close = data->fast_import_backchannel_pipe[0];
+       fastimport->preexec_cb = close_fd_prexec_cb;
       return start_command(fastimport);
 }

@@ -427,6 +469,11 @@ static int fetch_with_import(struct transport *transport,

       if (get_importer(transport, &fastimport))
               die("Couldn't run fast-import");

+
+       /* in the parent process we close both pipe ends. */
+       close(data->fast_import_backchannel_pipe[0]);
+       close(data->fast_import_backchannel_pipe[1]);
+
       for (i = 0; i < nr_heads; i++) {
               posn = to_fetch[i];
               if (posn->status & REF_STATUS_UPTODATE)

@@ -441,6 +488,7 @@ static int fetch_with_import(struct transport *transport,

       if (finish_command(&fastimport))
               die("Error while running fast-import");
+       free((void*)fastimport.argv[2]);
       free(fastimport.argv);
       fastimport.argv = NULL;

diff --git a/vcs-svn/svndump.c b/vcs-svn/svndump.c
index 2f0089f..b1fe03f 100644
--- a/vcs-svn/svndump.c
+++ b/vcs-svn/svndump.c

@@ -467,7 +467,14 @@ void svndump_read(const char *url)

 static void init()
 {
-       fast_export_init(REPORT_FILENO);
+       int report_fd;
+       char* back_fd_env = getenv("GIT_REPORT_FILENO");
+       if(!back_fd_env || sscanf(back_fd_env, "%d", &report_fd) != 1) {
+               warning("Cannot get cat-blob fd from environment, using default!");
+               report_fd = REPORT_FILENO;
+       }
+
+       fast_export_init(report_fd);
       strbuf_init(&dump_ctx.uuid, 4096);
       strbuf_init(&dump_ctx.url, 4096);
       strbuf_init(&rev_ctx.log, 4096);
--
1.7.9.5