commands: add debugdeltachain command

We have debug commands for displaying overall revlog statistics
(debugrevlog) and for dumping a revlog index (debugindex). As part
of investigating various aspects of revlog behavior and performance,
I found it important to have an understanding of how revlog
delta chains behave in practice.

This patch implements a "debugdeltachain" command. For each revision
in a revlog, it dumps information about the delta chain. Which delta
chain it is part of, length of the delta chain, distance since base
revision, info about base revision, size of the delta chain, etc. The
generic formatting facility is used, which means we can templatize
output and get machine readable output like JSON.

This command has already uncovered some weird history in
mozilla-central I didn't know about. So I think it's valuable.

mercurial/commands.py

@@ -2501,6 +2501,117 b' def debugindexdot(ui, repo, file_=None, '
             ui.write("\t%d -> %d\n" % (r.rev(pp[1]), i))
     ui.write("}\n")
 
+@command('debugdeltachain',
+    debugrevlogopts + formatteropts,
+    _('-c|-m|FILE'),
+    optionalrepo=True)
+def debugdeltachain(ui, repo, file_=None, **opts):
+    """dump information about delta chains in a revlog
+
+    Output can be templatized. Available template keywords are:
+
+       rev          revision number
+       chainid      delta chain identifier (numbered by unique base)
+       chainlen     delta chain length to this revision
+       prevrev      previous revision in delta chain
+       deltatype    role of delta / how it was computed
+       compsize     compressed size of revision
+       uncompsize   uncompressed size of revision
+       chainsize    total size of compressed revisions in chain
+       chainratio   total chain size divided by uncompressed revision size
+                    (new delta chains typically start at ratio 2.00)
+       lindist      linear distance from base revision in delta chain to end
+                    of this revision
+       extradist    total size of revisions not part of this delta chain from
+                    base of delta chain to end of this revision; a measurement
+                    of how much extra data we need to read/seek across to read
+                    the delta chain for this revision
+       extraratio   extradist divided by chainsize; another representation of
+                    how much unrelated data is needed to load this delta chain
+    """
+    r = cmdutil.openrevlog(repo, 'debugdeltachain', file_, opts)
+    index = r.index
+    generaldelta = r.version & revlog.REVLOGGENERALDELTA
+
+    def revinfo(rev):
+        iterrev = rev
+        e = index[iterrev]
+        chain = []
+        compsize = e[1]
+        uncompsize = e[2]
+        chainsize = 0
+
+        if generaldelta:
+            if e[3] == e[5]:
+                deltatype = 'p1'
+            elif e[3] == e[6]:
+                deltatype = 'p2'
+            elif e[3] == rev - 1:
+                deltatype = 'prev'
+            elif e[3] == rev:
+                deltatype = 'base'
+            else:
+                deltatype = 'other'
+        else:
+            if e[3] == rev:
+                deltatype = 'base'
+            else:
+                deltatype = 'prev'
+
+        while iterrev != e[3]:
+            chain.append(iterrev)
+            chainsize += e[1]
+            if generaldelta:
+                iterrev = e[3]
+            else:
+                iterrev -= 1
+            e = index[iterrev]
+        else:
+            chainsize += e[1]
+            chain.append(iterrev)
+
+        chain.reverse()
+        return compsize, uncompsize, deltatype, chain, chainsize
+
+    fm = ui.formatter('debugdeltachain', opts)
+
+    fm.plain('    rev  chain# chainlen     prev   delta       '
+             'size    rawsize  chainsize     ratio   lindist extradist '
+             'extraratio\n')
+
+    chainbases = {}
+    for rev in r:
+        comp, uncomp, deltatype, chain, chainsize = revinfo(rev)
+        chainbase = chain[0]
+        chainid = chainbases.setdefault(chainbase, len(chainbases) + 1)
+        basestart = r.start(chainbase)
+        revstart = r.start(rev)
+        lineardist = revstart + comp - basestart
+        extradist = lineardist - chainsize
+        try:
+            prevrev = chain[-2]
+        except IndexError:
+            prevrev = -1
+
+        chainratio = float(chainsize) / float(uncomp)
+        extraratio = float(extradist) / float(chainsize)
+
+        fm.startitem()
+        fm.write('rev chainid chainlen prevrev deltatype compsize '
+                 'uncompsize chainsize chainratio lindist extradist '
+                 'extraratio',
+                 '%7d %7d %8d %8d %7s %10d %10d %10d %9.5f %9d %9d %10.5f\n',
+                 rev, chainid, len(chain), prevrev, deltatype, comp,
+                 uncomp, chainsize, chainratio, lineardist, extradist,
+                 extraratio,
+                 rev=rev, chainid=chainid, chainlen=len(chain),
+                 prevrev=prevrev, deltatype=deltatype, compsize=comp,
+                 uncompsize=uncomp, chainsize=chainsize,
+                 chainratio=chainratio, lindist=lineardist,
+                 extradist=extradist, extraratio=extraratio)
+
+    fm.end()
+
 @command('debuginstall', [], '', norepo=True)
 def debuginstall(ui):
     '''test Mercurial installation

tests/test-completion.t

@@ -80,6 +80,7 b' Show debug commands if there are no othe'
   debugdag
   debugdata
   debugdate
+  debugdeltachain
   debugdirstate
   debugdiscovery
   debugextensions
@@ -243,6 +244,7 b' Show all commands + options'
   debugdag: tags, branches, dots, spaces
   debugdata: changelog, manifest, dir
   debugdate: extended
+  debugdeltachain: changelog, manifest, dir, template
   debugdirstate: nodates, datesort
   debugdiscovery: old, nonheads, ssh, remotecmd, insecure
   debugextensions: template

tests/test-debugcommands.t

@@ -44,6 +44,32 b' Test debugindex, with and without the --'
      rev flag   offset   length     size  .....   link     p1     p2                                   nodeid (re)
        0 0000        0        3        2   ....      0     -1     -1 b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3 (re)
 
+debugdelta chain basic output
+
+  $ hg debugdeltachain -m
+      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
+        0       1        1       -1    base         44         43         44   1.02326        44         0    0.00000
+
+  $ hg debugdeltachain -m -T '{rev} {chainid} {chainlen}\n'
+  0 1 1
+
+  $ hg debugdeltachain -m -Tjson
+  [
+   {
+    "chainid": 1,
+    "chainlen": 1,
+    "chainratio": 1.02325581395,
+    "chainsize": 44,
+    "compsize": 44,
+    "deltatype": "base",
+    "extradist": 0,
+    "extraratio": 0.0,
+    "lindist": 44,
+    "prevrev": -1,
+    "rev": 0,
+    "uncompsize": 43
+   }
+  ]
 
 Test max chain len
   $ cat >> $HGRCPATH << EOF

tests/test-help.t

@@ -812,6 +812,8 b' Test list of internal help commands'
                  description
    debugdata     dump the contents of a data file revision
    debugdate     parse and display a date
+   debugdeltachain
+                 dump information about delta chains in a revlog
    debugdirstate
                  show the contents of the current dirstate
    debugdiscovery