git-p4: auto-size the block

git-p4 originally would fetch changes in one query. On large repos this
could fail because of the limits that Perforce imposes on the number of
items returned and the number of queries in the database.

To fix this, git-p4 learned to query changes in blocks of 512 changes,
However, this can be very slow - if you have a few million changes,
with each chunk taking about a second, it can be an hour or so.

Although it's possible to tune this value manually with the
"--changes-block-size" option, it's far from obvious to ordinary users
that this is what needs doing.

This change alters the block size dynamically by looking for the
specific error messages returned from the Perforce server, and reducing
the block size if the error is seen, either to the limit reported by the
server, or to half the current block size.

That means we can start out with a very large block size, and then let
it automatically drop down to a value that works without error, while
still failing correctly if some other error occurs.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -47,8 +47,8 @@ def __str__(self):
 # Only labels/tags matching this will be imported/exported
 defaultLabelRegexp = r'[a-zA-Z0-9_\-.]+$'
 
-# Grab changes in blocks of this many revisions, unless otherwise requested
-defaultBlockSize = 512
+# The block size is reduced automatically if required
+defaultBlockSize = 1<<20
 
 p4_access_checked = False
 
@@ -958,7 +958,8 @@ def p4ChangesForPaths(depotPaths, changeRange, requestedBlockSize):
     changes = set()
 
     # Retrieve changes a block at a time, to prevent running
-    # into a MaxResults/MaxScanRows error from the server.
+    # into a MaxResults/MaxScanRows error from the server. If
+    # we _do_ hit one of those errors, turn down the block size
 
     while True:
         cmd = ['changes']
@@ -972,10 +973,24 @@ def p4ChangesForPaths(depotPaths, changeRange, requestedBlockSize):
         for p in depotPaths:
             cmd += ["%s...@%s" % (p, revisionRange)]
 
+        # fetch the changes
+        try:
+            result = p4CmdList(cmd, errors_as_exceptions=True)
+        except P4RequestSizeException as e:
+            if not block_size:
+                block_size = e.limit
+            elif block_size > e.limit:
+                block_size = e.limit
+            else:
+                block_size = max(2, block_size // 2)
+
+            if verbose: print("block size error, retrying with block size {0}".format(block_size))
+            continue
+        except P4Exception as e:
+            die('Error retrieving changes description ({0})'.format(e.p4ExitCode))
+
         # Insert changes in chronological order
-        for entry in reversed(p4CmdList(cmd)):
-            if entry.has_key('p4ExitCode'):
-                die('Error retrieving changes descriptions ({})'.format(entry['p4ExitCode']))
+        for entry in reversed(result):
             if not entry.has_key('change'):
                 continue
             changes.add(int(entry['change']))

t/t9818-git-p4-block.sh

@@ -129,6 +129,7 @@ test_expect_success 'Create a repo with multiple depot paths' '
 '
 
 test_expect_success 'Clone repo with multiple depot paths' '
+	test_when_finished cleanup_git &&
 	(
 		cd "$git" &&
 		git p4 clone --changes-block-size=4 //depot/pathA@all //depot/pathB@all \
@@ -138,6 +139,13 @@ test_expect_success 'Clone repo with multiple depot paths' '
 	)
 '
 
+test_expect_success 'Clone repo with self-sizing block size' '
+	test_when_finished cleanup_git &&
+	git p4 clone --changes-block-size=1000000 //depot@all --destination="$git" &&
+	git -C "$git" log --oneline >log &&
+	test_line_count \> 10 log
+'
+
 test_expect_success 'kill p4d' '
 	kill_p4d
 '