Merge #17887: bug-fix macos: give free bytes to F_PREALLOCATE

75163f4 bug-fix macos: give free bytes to F_PREALLOCATE (Karl-Johan Alm)

Pull request description:

  The macos manpage for `fcntl` (for `F_PEOFPOSMODE`) states:

  > Allocate from the physical end of file.  In this case, fst_length indicates the number of newly allocated bytes desired.

  This would result in the rev files being essentially pre-allocating 2x their necessary size (this is the case for block files as well, but these are flushed down to their right sizes every time) as they would pre-allocate `pos + length` **free** bytes, rather than allocating `length` bytes after `pos`, as expected.

  Fixes #17827.

ACKs for top commit:
  eriknylund:
    ACK 75163f4 built locally. All tests passing. Manual test as per my previous comment above on an older commit, using an APFS unencrypted disk image with 3 GB.
  laanwj:
    code review ACK 75163f4

Tree-SHA512: 105c8d56c20acad8febdf0583f1e5721b63376ace325a7a62c2e4b15a442c7131404ed604c32c0cda716791d7ca5aa9f5b6a774ff86e39838bc7e87ca3c42760

Author: laanwj

src/util/system.cpp

@@ -974,17 +974,19 @@ void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length) {
     SetEndOfFile(hFile);
 #elif defined(MAC_OSX)
     // OSX specific version
+    // NOTE: Contrary to other OS versions, the OSX version assumes that
+    // NOTE: offset is the size of the file.
     fstore_t fst;
     fst.fst_flags = F_ALLOCATECONTIG;
     fst.fst_posmode = F_PEOFPOSMODE;
     fst.fst_offset = 0;
-    fst.fst_length = (off_t)offset + length;
+    fst.fst_length = length; // mac os fst_length takes the # of free bytes to allocate, not desired file size
     fst.fst_bytesalloc = 0;
     if (fcntl(fileno(file), F_PREALLOCATE, &fst) == -1) {
         fst.fst_flags = F_ALLOCATEALL;
         fcntl(fileno(file), F_PREALLOCATE, &fst);
     }
-    ftruncate(fileno(file), fst.fst_length);
+    ftruncate(fileno(file), static_cast<off_t>(offset) + length);
 #else
     #if defined(__linux__)
     // Version using posix_fallocate