Merge branch 'jt/subprocess-handshake' into maint

Code cleanup.

* jt/subprocess-handshake:
  sub-process: refactor handshake to common function
  Documentation: migrate sub-process docs to header
  convert: add "status=delayed" to filter process protocol
  convert: refactor capabilities negotiation
  convert: move multiple file filter error handling to separate function
  convert: put the flags field before the flag itself for consistent style
  t0021: write "OUT <size>" only on success
  t0021: make debug log file name configurable
  t0021: keep filter log files on comparison

Documentation/gitattributes.txt

@@ -425,8 +425,8 @@ packet:          git< capability=clean
 packet:          git< capability=smudge
 packet:          git< 0000
 ------------------------
-Supported filter capabilities in version 2 are "clean" and
-"smudge".
+Supported filter capabilities in version 2 are "clean", "smudge",
+and "delay".
 
 Afterwards Git sends a list of "key=value" pairs terminated with
 a flush packet. The list will contain at least the filter command
@@ -512,12 +512,73 @@ the protocol then Git will stop the filter process and restart it
 with the next file that needs to be processed. Depending on the
 `filter.<driver>.required` flag Git will interpret that as error.
 
-After the filter has processed a blob it is expected to wait for
-the next "key=value" list containing a command. Git will close
+After the filter has processed a command it is expected to wait for
+a "key=value" list containing the next command. Git will close
 the command pipe on exit. The filter is expected to detect EOF
 and exit gracefully on its own. Git will wait until the filter
 process has stopped.
 
+Delay
+^^^^^
+
+If the filter supports the "delay" capability, then Git can send the
+flag "can-delay" after the filter command and pathname. This flag
+denotes that the filter can delay filtering the current blob (e.g. to
+compensate network latencies) by responding with no content but with
+the status "delayed" and a flush packet.
+------------------------
+packet:          git> command=smudge
+packet:          git> pathname=path/testfile.dat
+packet:          git> can-delay=1
+packet:          git> 0000
+packet:          git> CONTENT
+packet:          git> 0000
+packet:          git< status=delayed
+packet:          git< 0000
+------------------------
+
+If the filter supports the "delay" capability then it must support the
+"list_available_blobs" command. If Git sends this command, then the
+filter is expected to return a list of pathnames representing blobs
+that have been delayed earlier and are now available.
+The list must be terminated with a flush packet followed
+by a "success" status that is also terminated with a flush packet. If
+no blobs for the delayed paths are available, yet, then the filter is
+expected to block the response until at least one blob becomes
+available. The filter can tell Git that it has no more delayed blobs
+by sending an empty list. As soon as the filter responds with an empty
+list, Git stops asking. All blobs that Git has not received at this
+point are considered missing and will result in an error.
+
+------------------------
+packet:          git> command=list_available_blobs
+packet:          git> 0000
+packet:          git< pathname=path/testfile.dat
+packet:          git< pathname=path/otherfile.dat
+packet:          git< 0000
+packet:          git< status=success
+packet:          git< 0000
+------------------------
+
+After Git received the pathnames, it will request the corresponding
+blobs again. These requests contain a pathname and an empty content
+section. The filter is expected to respond with the smudged content
+in the usual way as explained above.
+------------------------
+packet:          git> command=smudge
+packet:          git> pathname=path/testfile.dat
+packet:          git> 0000
+packet:          git> 0000  # empty content!
+packet:          git< status=success
+packet:          git< 0000
+packet:          git< SMUDGED_CONTENT
+packet:          git< 0000
+packet:          git< 0000  # empty list, keep "status=success" unchanged!
+------------------------
+
+Example
+^^^^^^^
+
 A long running filter demo implementation can be found in
 `contrib/long-running-filter/example.pl` located in the Git
 core repository. If you develop your own long running filter

Documentation/technical/api-sub-process.txt

@@ -1,59 +0,0 @@
-sub-process API
-===============
-
-The sub-process API makes it possible to run background sub-processes
-for the entire lifetime of a Git invocation. If Git needs to communicate
-with an external process multiple times, then this can reduces the process
-invocation overhead. Git and the sub-process communicate through stdin and
-stdout.
-
-The sub-processes are kept in a hashmap by command name and looked up
-via the subprocess_find_entry function.  If an existing instance can not
-be found then a new process should be created and started.  When the
-parent git command terminates, all sub-processes are also terminated.
-
-This API is based on the run-command API.
-
-Data structures
----------------
-
-* `struct subprocess_entry`
-
-The sub-process structure.  Members should not be accessed directly.
-
-Types
------
-
-'int(*subprocess_start_fn)(struct subprocess_entry *entry)'::
-
-	User-supplied function to initialize the sub-process.  This is
-	typically used to negotiate the interface version and capabilities.
-
-
-Functions
----------
-
-`cmd2process_cmp`::
-
-	Function to test two subprocess hashmap entries for equality.
-
-`subprocess_start`::
-
-	Start a subprocess and add it to the subprocess hashmap.
-
-`subprocess_stop`::
-
-	Kill a subprocess and remove it from the subprocess hashmap.
-
-`subprocess_find_entry`::
-
-	Find a subprocess in the subprocess hashmap.
-
-`subprocess_get_child_process`::
-
-	Get the underlying `struct child_process` from a subprocess.
-
-`subprocess_read_status`::
-
-	Helper function to read packets looking for the last "status=<foo>"
-	key/value pair.