fs: use ftruncate in AllocateFileRange on OpenBSD #32645

1. 
   theStack commented at 1:41 pm on May 30, 2025: contributor

   Closes #32643.

   Tested on OpenBSD 7.7 (amd64) by running a signet IBD, with the following patch on top to ensure the fallback isn’t executed anymore:

    0diff --git a/src/util/fs_helpers.cpp b/src/util/fs_helpers.cpp
    1index 84ac3690c9..c6dff2d29d 100644
    2--- a/src/util/fs_helpers.cpp
    3+++ b/src/util/fs_helpers.cpp
    4@@ -208,6 +208,7 @@ void AllocateFileRange(FILE* file, unsigned int offset, unsigned int length)
    5     // OpenBSD doesn't have fallocate or posix_fallocate, use ftruncate
    6     if (0 == ftruncate(fileno(file), nEndPos)) return;
    7 #endif
    8+    assert(false);
    9     // Fallback version
   10     // TODO: just write one byte per block
   11     static const char buf[65536] = {};
   

   Didn’t run any performance tests, happy to take inputs on how to best measure any potential difference.

   For another reference, here’s an OpenBSD ports patch for kitty that replaces posix_fallocate with ftruncate: https://github.com/openbsd/ports/blob/master/x11/kitty/patches/patch-glfw_backend_utils_c

2. fs: use ftruncate in AllocateFileRange on OpenBSD

   Closes issue #32643.

   9982d525c5
3. 
   DrahtBot commented at 1:41 pm on May 30, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32645.

   Reviews

   See the guideline for information on the review process. A summary of reviews will appear here.

4. in src/util/fs_helpers.cpp:201 in 9982d525c5

   199@@ -200,10 +200,13 @@ void AllocateFileRange(FILE* file, unsigned int offset, unsigned int length)
   200     }
   201     ftruncate(fileno(file), static_cast<off_t>(offset) + length);
   

   ---

   ---

   

   luke-jr commented at 9:19 pm on June 6, 2025:

   We’re already calling fallocate here… seems like this whole #if mess should be sorted out.
5. in src/util/fs_helpers.cpp:207 in 9982d525c5

   199@@ -200,10 +200,13 @@ void AllocateFileRange(FILE* file, unsigned int offset, unsigned int length)
   200     }
   201     ftruncate(fileno(file), static_cast<off_t>(offset) + length);
   202 #else
   203+    off_t nEndPos = (off_t)offset + length;
   204 #if defined(HAVE_POSIX_FALLOCATE)
   205     // Version using posix_fallocate
   206-    off_t nEndPos = (off_t)offset + length;
   207     if (0 == posix_fallocate(fileno(file), 0, nEndPos)) return;
   208+#elif defined(__OpenBSD__)
   

   ---

   ---

   

   luke-jr commented at 9:20 pm on June 6, 2025:

   ftruncate isn’t OpenBSD-specific. We should just check its availability and use it on platforms that support it (after preferred methods are ruled out)
6. luke-jr changes_requested

Contributors
theStack DrahtBot luke-jr