revlog: use existing file handle when reading during _addrevision

_addrevision() may need to read from revlogs as part of computing
deltas. Previously, we would flush existing file handles and open
a new, short-lived file handle to perform the reading.

If we have an existing file handle, it seems logical to reuse it
for reading instead of opening a new file handle. This patch
makes that the new behavior.

After this patch, revlog files are only reopened when adding
revisions if the revlog is switched from inline to non-inline.

On Linux when unbundling a bundle of the mozilla-central repo, this
patch has the following impact on system call counts:

Call     Before     After       Delta
write    827,639    673,390   -154,249
open     700,103    684,089    -16,014
read      74,489     74,489          0
fstat    493,924    461,896    -32,028
close    249,131    233,117    -16,014
stat     242,001    242,001          0
lstat     18,676     18,676          0
lseek     20,268     20,268          0
ioctl     14,652     13,173     -1,479
TOTAL  3,180,758  2,930,679   -250,079

It's worth noting that many of the open() calls fail due to missing
files. That's why there are many more open() calls than close().

Despite the significant system call reduction, this change does not
seem to have a significant performance impact on Linux.

On Windows 10 (not a VM, on a SSD), this patch appears to reduce
unbundle time for mozilla-central from ~960s to ~920s. This isn't
as significant as I was hoping. But a decrease it is nonetheless.
Still, Windows unbundle performance is still >2x slower than Linux.

Despite the lack of significant gains, fewer system calls is fewer
system calls. If nothing else, this will narrow the focus of potential
areas to optimize in the future.

mercurial/revlog.py

@@ -1289,10 +1289,6 b' class revlog(object):'
         def buildtext():
             if btext[0] is not None:
                 return btext[0]
-            # flush any pending writes here so we can read it in revision
-            if dfh:
-                dfh.flush()
-            ifh.flush()
             baserev = cachedelta[0]
             delta = cachedelta[1]
             # special case deltas which replace entire base; no need to decode
@@ -1303,7 +1299,11 b' class revlog(object):'
                                                        len(delta) - hlen):
                 btext[0] = delta[hlen:]
             else:
-                basetext = self.revision(self.node(baserev))
+                if self._inline:
+                    fh = ifh
+                else:
+                    fh = dfh
+                basetext = self.revision(self.node(baserev), _df=fh)
                 btext[0] = mdiff.patch(basetext, delta)
             try:
                 self.checkhash(btext[0], p1, p2, node)
@@ -1327,7 +1327,11 b' class revlog(object):'
                     header = mdiff.replacediffheader(self.rawsize(rev), len(t))
                     delta = header + t
                 else:
-                    ptext = self.revision(self.node(rev))
+                    if self._inline:
+                        fh = ifh
+                    else:
+                        fh = dfh
+                    ptext = self.revision(self.node(rev), _df=fh)
                     delta = mdiff.textdiff(ptext, t)
             data = self.compress(delta)
             l = len(data[1]) + len(data[0])