deltas: add code to display information about the result of `finddeltainfo`

I have been looking into performance issue around pull and getting more
information about the computation and time involved into applying each revision
is very useful. There will be various way to use this new output, so I am
introducing the code first.

mercurial/revlogutils/deltas.py

@@ -20,6 +20,9 b' from .constants import ('
     COMP_MODE_DEFAULT,
     COMP_MODE_INLINE,
     COMP_MODE_PLAIN,
+    KIND_CHANGELOG,
+    KIND_FILELOG,
+    KIND_MANIFESTLOG,
     REVIDX_ISCENSORED,
     REVIDX_RAWTEXT_CHANGING_FLAGS,
 )
@@ -928,8 +931,9 b' def _rawgroups(revlog, p1, p2, cachedelt'
 
 
 class deltacomputer:
-    def __init__(self, revlog):
+    def __init__(self, revlog, write_debug=None):
         self.revlog = revlog
+        self._write_debug = write_debug
 
     def buildtext(self, revinfo, fh):
         """Builds a fulltext version of a revision
@@ -1083,6 +1087,15 b' class deltacomputer:'
         if revinfo.flags & REVIDX_RAWTEXT_CHANGING_FLAGS:
             return self._fullsnapshotinfo(fh, revinfo, target_rev)
 
+        if self._write_debug is not None:
+            start = util.timer()
+
+        # count the number of different delta we tried (for debug purpose)
+        dbg_try_count = 0
+        # count the number of "search round" we did. (for debug purpose)
+        dbg_try_rounds = 0
+        dbg_type = b'unknown'
+
         cachedelta = revinfo.cachedelta
         p1 = revinfo.p1
         p2 = revinfo.p2
@@ -1090,11 +1103,23 b' class deltacomputer:'
 
         deltainfo = None
         p1r, p2r = revlog.rev(p1), revlog.rev(p2)
+
+        if self._write_debug is not None:
+            if p1r != nullrev:
+                p1_chain_len = revlog._chaininfo(p1r)[0]
+            else:
+                p1_chain_len = -1
+            if p2r != nullrev:
+                p2_chain_len = revlog._chaininfo(p2r)[0]
+            else:
+                p2_chain_len = -1
+
         groups = _candidategroups(
             self.revlog, revinfo.textlen, p1r, p2r, cachedelta
         )
         candidaterevs = next(groups)
         while candidaterevs is not None:
+            dbg_try_rounds += 1
             nominateddeltas = []
             if deltainfo is not None:
                 # if we already found a good delta,
@@ -1105,6 +1130,7 b' class deltacomputer:'
                     continue
                 if candidaterev >= target_rev:
                     continue
+                dbg_try_count += 1
                 candidatedelta = self._builddeltainfo(revinfo, candidaterev, fh)
                 if candidatedelta is not None:
                     if isgooddeltainfo(self.revlog, candidatedelta, revinfo):
@@ -1117,7 +1143,68 b' class deltacomputer:'
                 candidaterevs = next(groups)
 
         if deltainfo is None:
+            dbg_type = b"full"
             deltainfo = self._fullsnapshotinfo(fh, revinfo, target_rev)
+        elif deltainfo.snapshotdepth:
+            dbg_type = b"snapshot"
+        else:
+            dbg_type = b"delta"
+
+        if self._write_debug is not None:
+            end = util.timer()
+            dbg = {
+                'duration': end - start,
+                'revision': target_rev,
+                'search_round_count': dbg_try_rounds,
+                'delta_try_count': dbg_try_count,
+                'type': dbg_type,
+                'p1-chain-len': p1_chain_len,
+                'p2-chain-len': p2_chain_len,
+            }
+            if deltainfo.snapshotdepth is not None:
+                dbg['snapshot-depth'] = deltainfo.snapshotdepth
+            else:
+                dbg['snapshot-depth'] = 0
+            target_revlog = b"UNKNOWN"
+            target_type = self.revlog.target[0]
+            target_key = self.revlog.target[1]
+            if target_type == KIND_CHANGELOG:
+                target_revlog = b'CHANGELOG:'
+            elif target_type == KIND_MANIFESTLOG:
+                target_revlog = b'MANIFESTLOG:'
+                if target_key:
+                    target_revlog += b'%s:' % target_key
+            elif target_type == KIND_FILELOG:
+                target_revlog = b'FILELOG:'
+                if target_key:
+                    target_revlog += b'%s:' % target_key
+            dbg['target-revlog'] = target_revlog
+
+            msg = (
+                b"DBG-DELTAS:"
+                b" %-12s"
+                b" rev=%d:"
+                b" search-rounds=%d"
+                b" try-count=%d"
+                b" - delta-type=%-6s"
+                b" snap-depth=%d"
+                b" - p1-chain-length=%d"
+                b" p2-chain-length=%d"
+                b" - duration=%f"
+                b"\n"
+            )
+            msg %= (
+                dbg["target-revlog"],
+                dbg["revision"],
+                dbg["search_round_count"],
+                dbg["delta_try_count"],
+                dbg["type"],
+                dbg["snapshot-depth"],
+                dbg["p1-chain-len"],
+                dbg["p2-chain-len"],
+                dbg["duration"],
+            )
+            self._write_debug(msg)
         return deltainfo