From: David Barr <david.b...@cordelta.com>

New objects written by fast-import are not available immediately.
Until a checkpoint has been started and finishes writing the pack
index, any new blobs will not be accessible using standard git tools.

So introduce a new way to access them: a "cat-blob" command in the
command stream requests for fast-import to print a blob to stdout or a
file descriptor specified by the argument to --cat-blob-fd.  The value
for cat-blob-fd cannot be specified in the stream because that would
be a layering violation: the decision of where to direct a stream has
to be made when fast-import is started anyway, so we might as well
make the stream format is independent of that detail.

Output uses the same format as "git cat-file --batch".

Thanks to Sverre Rabbelier and Sam Vilain for guidance in designing
the protocol.

Based-on-patch-by: Jonathan Nieder <jrnie...@gmail.com>
Signed-off-by: David Barr <david.b...@cordelta.com>
Acked-by: Ramkumar Ramachandra <artag...@gmail.com>
Signed-off-by: Jonathan Nieder <jrnie...@gmail.com>
---
If you use this, you might want to do something like:

        blob 
        mark :1
        data <<EOT
        testing 1 2 3
        EOT
        cat-blob :1

before proceeding with the rest of the stream.  This allows wiring
mistakes to be caught early.

 Documentation/git-fast-import.txt |   41 ++++++++
 fast-import.c                     |   95 ++++++++++++++++++
 t/t9300-fast-import.sh            |  193 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 327 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-fast-import.txt 
b/Documentation/git-fast-import.txt
index 3bf04e3..be444da 100644
--- a/Documentation/git-fast-import.txt
+++ b/Documentation/git-fast-import.txt
@@ -92,6 +92,11 @@ OPTIONS
        --(no-)-relative-marks= with the --(import|export)-marks=
        options.
 
+--cat-blob-fd=<fd>::
+       Specify the file descriptor that will be written to
+       when the `cat-blob` command is encountered in the stream.
+       The default behaviour is to write to `stdout`.
+
 --export-pack-edges=<file>::
        After creating a packfile, print a line of data to
        <file> listing the filename of the packfile and the last
@@ -320,6 +325,11 @@ and control the current import process.  More detailed 
discussion
        standard output.  This command is optional and is not needed
        to perform an import.
 
+`cat-blob`::
+       Causes fast-import to print a blob in 'cat-file --batch'
+       format to the file descriptor set with `--cat-blob-fd` or
+       `stdout` if unspecified.
+
 `feature`::
        Require that fast-import supports the specified feature, or
        abort if it does not.
@@ -872,6 +882,29 @@ Placing a `progress` command immediately after a 
`checkpoint` will
 inform the reader when the `checkpoint` has been completed and it
 can safely access the refs that fast-import updated.
 
+`cat-blob`
+~~~~~~~~~~
+Causes fast-import to print a blob to a file descriptor previously
+arranged with the `--cat-blob-fd` argument.  The command otherwise
+has no impact on the current import; its main purpose is to
+retrieve blobs that may be in fast-import's memory but not
+accessible from the target repository.
+
+....
+       'cat-blob' SP <dataref> LF
+....
+
+The `<dataref>` can be either a mark reference (`:<idnum>`)
+set previously or a full 40-byte SHA-1 of a Git blob, preexisting or
+ready to be written.
+
+output uses the same format as `git cat-file --batch`:
+
+====
+       <sha1> SP 'blob' SP <size> LF
+       <contents> LF
+====
+
 `feature`
 ~~~~~~~~~
 Require that fast-import supports the specified feature, or abort if
@@ -898,6 +931,13 @@ import-marks::
        second, an --import-marks= command-line option overrides
        any "feature import-marks" command in the stream.
 
+cat-blob::
+       Ignored.  Versions of fast-import not supporting the
+       "cat-blob" command will exit with a message indicating so.
+       This lets the import error out early with a clear message,
+       rather than wasting time on the early part of an import
+       before the unsupported command is detected.
+
 `option`
 ~~~~~~~~
 Processes the specified option so that git fast-import behaves in a
@@ -923,6 +963,7 @@ not be passed as option:
 * date-format
 * import-marks
 * export-marks
+* cat-blob-fd
 * force
 
 Crash Reports
diff --git a/fast-import.c b/fast-import.c
index 959afef..88547c6 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -55,6 +55,8 @@ Format of STDIN stream:
     ('from' sp committish lf)?
     lf?;
 
+  cat_blob ::= 'cat-blob' sp (hexsha1 | idnum) lf;
+
   checkpoint ::= 'checkpoint' lf
     lf?;
 
@@ -361,6 +363,9 @@ static uintmax_t next_mark;
 static struct strbuf new_data = STRBUF_INIT;
 static int seen_data_command;
 
+/* Where to write output of cat-blob commands */
+static int cat_blob_fd = STDOUT_FILENO;
+
 static void parse_argv(void);
 
 static void write_branch_report(FILE *rpt, struct branch *b)
@@ -2689,6 +2694,79 @@ static void parse_reset_branch(void)
                unread_command_buf = 1;
 }
 
+static void cat_blob_write(const char *buf, unsigned long size)
+{
+       if (write_in_full(cat_blob_fd, buf, size) != size)
+               die_errno("Write to frontend failed");
+}
+
+static void cat_blob(struct object_entry *oe, unsigned char sha1[20])
+{
+       struct strbuf line = STRBUF_INIT;
+       unsigned long size;
+       enum object_type type = 0;
+       char *buf;
+
+       if (!oe || oe->pack_id == MAX_PACK_ID) {
+               buf = read_sha1_file(sha1, &type, &size);
+       } else {
+               type = oe->type;
+               buf = gfi_unpack_entry(oe, &size);
+       }
+
+       /*
+        * Output based on batch_one_object() from cat-file.c.
+        */
+       if (type <= 0) {
+               strbuf_reset(&line);
+               strbuf_addf(&line, "%s missing\n", sha1_to_hex(sha1));
+               cat_blob_write(line.buf, line.len);
+               free(buf);
+               return;
+       }
+       if (!buf)
+               die("Can't read object %s", sha1_to_hex(sha1));
+       if (type != OBJ_BLOB)
+               die("Object %s is a %s but a blob was expected.",
+                   sha1_to_hex(sha1), typename(type));
+       strbuf_reset(&line);
+       strbuf_addf(&line, "%s %s %lu\n", sha1_to_hex(sha1),
+                                               typename(type), size);
+       cat_blob_write(line.buf, line.len);
+       cat_blob_write(buf, size);
+       cat_blob_write("\n", 1);
+       free(buf);
+}
+
+static void parse_cat_blob(void)
+{
+       const char *p;
+       struct object_entry *oe = oe;
+       unsigned char sha1[20];
+
+       /* cat-blob SP <object> LF */
+       p = command_buf.buf + strlen("cat-blob ");
+       if (*p == ':') {
+               char *x;
+               oe = find_mark(strtoumax(p + 1, &x, 10));
+               if (x == p + 1)
+                       die("Invalid mark: %s", command_buf.buf);
+               if (!oe)
+                       die("Unknown mark: %s", command_buf.buf);
+               if (*x)
+                       die("Garbage after mark: %s", command_buf.buf);
+               hashcpy(sha1, oe->idx.sha1);
+       } else {
+               if (get_sha1_hex(p, sha1))
+                       die("Invalid SHA1: %s", command_buf.buf);
+               if (p[40])
+                       die("Garbage after SHA1: %s", command_buf.buf);
+               oe = find_object(sha1);
+       }
+
+       cat_blob(oe, sha1);
+}
+
 static void parse_checkpoint(void)
 {
        if (object_count) {
@@ -2771,6 +2849,14 @@ static void option_export_marks(const char *marks)
        export_marks_file = make_fast_import_path(marks);
 }
 
+static void option_cat_blob_fd(const char *fd)
+{
+       unsigned long n = ulong_arg("--cat-blob-fd", fd);
+       if (n > (unsigned long) INT_MAX)
+               die("--cat-blob-fd cannot exceed %d", INT_MAX);
+       cat_blob_fd = (int) n;
+}
+
 static void option_export_pack_edges(const char *edges)
 {
        if (pack_edges)
@@ -2824,6 +2910,8 @@ static int parse_one_feature(const char *feature, int 
from_stream)
                option_import_marks(feature + 13, from_stream);
        } else if (!prefixcmp(feature, "export-marks=")) {
                option_export_marks(feature + 13);
+       } else if (!strcmp(feature, "cat-blob")) {
+               ; /* Don't die - this feature is supported */
        } else if (!prefixcmp(feature, "relative-marks")) {
                relative_marks_paths = 1;
        } else if (!prefixcmp(feature, "no-relative-marks")) {
@@ -2918,6 +3006,11 @@ static void parse_argv(void)
                if (parse_one_feature(a + 2, 0))
                        continue;
 
+               if (!prefixcmp(a + 2, "cat-blob-fd=")) {
+                       option_cat_blob_fd(a + 2 + strlen("cat-blob-fd="));
+                       continue;
+               }
+
                die("unknown option %s", a);
        }
        if (i != global_argc)
@@ -2969,6 +3062,8 @@ int main(int argc, const char **argv)
                        parse_new_tag();
                else if (!prefixcmp(command_buf.buf, "reset "))
                        parse_reset_branch();
+               else if (!prefixcmp(command_buf.buf, "cat-blob "))
+                       parse_cat_blob();
                else if (!strcmp("checkpoint", command_buf.buf))
                        parse_checkpoint();
                else if (!prefixcmp(command_buf.buf, "progress "))
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 2c27da6..3e2741b 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -23,11 +23,18 @@ file5_data='an inline file.
 file6_data='#!/bin/sh
 echo "$@"'
 
+>empty
+
 ###
 ### series A
 ###
 
 test_tick
+
+test_expect_success 'empty stream succeeds' '
+       git fast-import </dev/null
+'
+
 cat >input <<INPUT_END
 blob
 mark :2
@@ -1501,6 +1508,190 @@ test_expect_success 'R: feature no-relative-marks 
should be honoured' '
     test_cmp marks.new non-relative.out
 '
 
+test_expect_success 'R: feature cat-blob supported' '
+       echo "feature cat-blob" |
+       git fast-import
+'
+
+test_expect_success 'R: cat-blob-fd must be a nonnegative integer' '
+       test_must_fail git fast-import --cat-blob-fd=-1 </dev/null
+'
+
+test_expect_success 'R: print old blob' '
+       blob=$(echo "yes it can" | git hash-object -w --stdin) &&
+       cat >expect <<-EOF &&
+       ${blob} blob 11
+       yes it can
+
+       EOF
+       echo "cat-blob $blob" |
+       git fast-import --cat-blob-fd=6 6>actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'R: in-stream cat-blob-fd not respected' '
+       echo hello >greeting &&
+       blob=$(git hash-object -w greeting) &&
+       cat >expect <<-EOF &&
+       ${blob} blob 6
+       hello
+
+       EOF
+       git fast-import --cat-blob-fd=3 3>actual.3 >actual.1 <<-EOF &&
+       cat-blob $blob
+       EOF
+       test_cmp expect actual.3 &&
+       test_cmp empty actual.1 &&
+       git fast-import 3>actual.3 >actual.1 <<-EOF &&
+       option cat-blob-fd=3
+       cat-blob $blob
+       EOF
+       test_cmp empty actual.3 &&
+       test_cmp expect actual.1
+'
+
+test_expect_success 'R: print new blob' '
+       blob=$(echo "yep yep yep" | git hash-object --stdin) &&
+       cat >expect <<-EOF &&
+       ${blob} blob 12
+       yep yep yep
+
+       EOF
+       git fast-import --cat-blob-fd=6 6>actual <<-\EOF &&
+       blob
+       mark :1
+       data <<BLOB_END
+       yep yep yep
+       BLOB_END
+       cat-blob :1
+       EOF
+       test_cmp expect actual
+'
+
+test_expect_success 'R: print new blob by sha1' '
+       blob=$(echo "a new blob named by sha1" | git hash-object --stdin) &&
+       cat >expect <<-EOF &&
+       ${blob} blob 25
+       a new blob named by sha1
+
+       EOF
+       git fast-import --cat-blob-fd=6 6>actual <<-EOF &&
+       blob
+       data <<BLOB_END
+       a new blob named by sha1
+       BLOB_END
+       cat-blob $blob
+       EOF
+       test_cmp expect actual
+'
+
+test_expect_success 'setup: big file' '
+       (
+               echo "the quick brown fox jumps over the lazy dog" >big &&
+               for i in 1 2 3
+               do
+                       cat big big big big >bigger &&
+                       cat bigger bigger bigger bigger >big ||
+                       exit
+               done
+       )
+'
+
+test_expect_success 'R: print two blobs to stdout' '
+       blob1=$(git hash-object big) &&
+       blob1_len=$(wc -c <big) &&
+       blob2=$(echo hello | git hash-object --stdin) &&
+       {
+               echo ${blob1} blob $blob1_len &&
+               cat big &&
+               cat <<-EOF
+
+               ${blob2} blob 6
+               hello
+
+               EOF
+       } >expect &&
+       {
+               cat <<-\END_PART1 &&
+                       blob
+                       mark :1
+                       data <<data_end
+               END_PART1
+               cat big &&
+               cat <<-\EOF
+                       data_end
+                       blob
+                       mark :2
+                       data <<data_end
+                       hello
+                       data_end
+                       cat-blob :1
+                       cat-blob :2
+               EOF
+       } |
+       git fast-import >actual &&
+       test_cmp expect actual
+'
+
+test_expect_success 'setup: have pipes?' '
+       rm -f frob &&
+       if mkfifo frob
+       then
+               test_set_prereq PIPE
+       fi
+'
+
+test_expect_success PIPE 'R: copy using cat-file' '
+       expect_id=$(git hash-object big) &&
+       expect_len=$(wc -c <big) &&
+       echo $expect_id blob $expect_len >expect.response &&
+
+       rm -f blobs &&
+       cat >frontend <<-\FRONTEND_END &&
+       #!/bin/sh
+       cat <<EOF &&
+       feature cat-blob
+       blob
+       mark :1
+       data <<BLOB
+       EOF
+       cat big
+       cat <<EOF
+       BLOB
+       cat-blob :1
+       EOF
+
+       read blob_id type size <&3 &&
+       echo "$blob_id $type $size" >response &&
+       dd if=/dev/stdin of=blob bs=$size count=1 <&3 &&
+       read newline <&3 &&
+
+       cat <<EOF &&
+       commit refs/heads/copied
+       committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+       data <<COMMIT
+       copy big file as file3
+       COMMIT
+       M 644 inline file3
+       data <<BLOB
+       EOF
+       cat blob &&
+       cat <<EOF
+       BLOB
+       EOF
+       FRONTEND_END
+
+       mkfifo blobs &&
+       (
+               export GIT_COMMITTER_NAME GIT_COMMITTER_EMAIL 
GIT_COMMITTER_DATE &&
+               sh frontend 3<blobs |
+               git fast-import --cat-blob-fd=3 3>blobs
+       ) &&
+       git show copied:file3 >actual &&
+       test_cmp expect.response response &&
+       test_cmp big actual
+'
+
 cat >input << EOF
 option git quiet
 blob
@@ -1509,8 +1700,6 @@ hi
 
 EOF
 
-touch empty
-
 test_expect_success 'R: quiet option results in no stats being output' '
     cat input | git fast-import 2> output &&
     test_cmp empty output
-- 
1.7.2.3


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