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.