largefiles: don't verify largefile hashes on servers when processing statlfile

When changesets referencing largefiles are pushed then the corresponding
largefiles will be pushed too - unless the target already has them. The client
will use statlfile to make sure it only sends largefiles that the target
doesn't have. The server would however on every statlfile check that the
content of the largefile had the expected hash. What should be cheap thus
became an expensive operation that trashed the disk and the cache.

Largefile hashes are already checked by putlfile before being stored on the
server. A server should thus be able to keep its largefile store free of
errors - even more than it can keep revlogs free of errors. Verification should
happen when running 'hg verify' locally on the server. Rehashing every
largefile on every remote stat is too expensive.

Clients will also stat lfiles before downloading them. When the server verified
the hash in stat it meant that it had to read the file twice to serve it.

With this change the server will assume its own hashes are ok without checking
them on every statlfile.

Some consequences of this change:
- in case of server side corruption the problem will be detected by the
  existing check on the client side - not on server side
- clients that could upload an uncorrupted largefile when pushing will no
  longer magically heal the server (and break hardlinks) - a client will now
  only upload its uncorrupted files after the corrupted file has been removed
  on the server side
- client side verify will no longer report corruption in files it doesn't have

(Issue3123 discussed related problems - and how they have been fixed.)

hgext/largefiles/proto.py

@@ -63,18 +63,16 b' def getlfile(repo, proto, sha):'
     return wireproto.streamres(generator())
 
 def statlfile(repo, proto, sha):
-    '''Return '2\n' if the largefile is missing, '1\n' if it has a
-    mismatched checksum, or '0\n' if it is in good condition'''
+    '''Return '2\n' if the largefile is missing, '0\n' if it seems to be in
+    good condition.
+
+    The value 1 is reserved for mismatched checksum, but that is too expensive
+    to be verified on every stat and must be caught be running 'hg verify'
+    server side.'''
     filename = lfutil.findfile(repo, sha)
     if not filename:
         return '2\n'
-    fd = None
-    try:
-        fd = open(filename, 'rb')
-        return lfutil.hexsha1(fd) == sha and '0\n' or '1\n'
-    finally:
-        if fd:
-            fd.close()
+    return '0\n'
 
 def wirereposetup(ui, repo):
     class lfileswirerepository(repo.__class__):

tests/test-largefiles.t

@@ -1593,7 +1593,7 b' Corrupt the cached largefile in r7 and m'
   abort: remotestore: could not put $TESTTMP/r7/.hg/largefiles/4cdac4d8b084d0b599525cf732437fb337d422a8 to remote store http://localhost:$HGPORT1/ (glob)
   [255]
   $ mv 4cdac4d8b084d0b599525cf732437fb337d422a8 r7/.hg/largefiles/4cdac4d8b084d0b599525cf732437fb337d422a8
-Push of file that exists on server but is corrupted - magic healing is nice ... but too magic
+Push of file that exists on server but is corrupted - magic healing would be nice ... but too magic
   $ echo "server side corruption" > empty/.hg/largefiles/4cdac4d8b084d0b599525cf732437fb337d422a8
   $ hg push -R r7 http://localhost:$HGPORT1
   pushing to http://localhost:$HGPORT1/
@@ -1604,7 +1604,7 b' Push of file that exists on server but i'
   remote: adding file changes
   remote: added 2 changesets with 2 changes to 2 files
   $ cat empty/.hg/largefiles/4cdac4d8b084d0b599525cf732437fb337d422a8
-  c2
+  server side corruption
   $ rm -rf empty
 
 Push a largefiles repository to a served empty repository
@@ -1670,8 +1670,9 b' largefiles pulled on update - a largefil'
   $ echo corruption > empty/.hg/largefiles/02a439e5c31c526465ab1a0ca1f431f76b827b90
   $ hg -R http-clone up --config largefiles.usercache=http-clone-usercache
   getting changed largefiles
-  abort: remotestore: largefile 02a439e5c31c526465ab1a0ca1f431f76b827b90 is invalid
-  [255]
+  f1: data corruption (expected 02a439e5c31c526465ab1a0ca1f431f76b827b90, got 6a7bb2556144babe3899b25e5428123735bb1e27)
+  0 largefiles updated, 0 removed
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg -R http-clone st
   ! f1
   $ [ ! -f http-clone/.hg/largefiles/02a439e5c31c526465ab1a0ca1f431f76b827b90 ]
@@ -1684,9 +1685,7 b' largefiles pulled on update - a largefil'
   checking files
   1 files, 1 changesets, 1 total revisions
   searching 1 changesets for largefiles
-  changeset 0:cf03e5bb9936: f1: contents differ
   verified contents of 1 revisions of 1 largefiles
-  [1]
   $ hg -R http-clone up -Cqr null
 
 largefiles pulled on update - no server side problems: