Subject: [PATCH 13/14] rust: add a small wrapper around the hashfile code

From: "brian m. carlson" <[email-masked]>
Date: Mon, 27 Oct 2025 00:44:03 +0000

Our new binary loose object map code avoids needing to be intimately
involved with file handling by simply writing data to an object
implement Write.  This makes it very easy to test by writing to a Cursor
wrapping a Vec for tests, and thus decouples it from intimate knowledge
about how we handle files.

However, we will actually want to write our data to an actual file,
since that's the most practical way to persist data.  Implement a
wrapper around the hashfile code that implements the Write trait so that
we can write our loose object map into a file.

Signed-off-by: brian m. carlson <[email-masked]>
---
 Makefile         |  1 +
 src/csum_file.rs | 81 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs       |  1 +
 src/meson.build  |  1 +
 4 files changed, 84 insertions(+)
 create mode 100644 src/csum_file.rs

diff --git a/Makefile b/Makefile
index 2081b13780..8eb31aeed2 100644
--- a/Makefile
+++ b/Makefile
@@ -1521,6 +1521,7 @@ CLAR_TEST_OBJS += $(UNIT_TEST_DIR)/unit-test.o
 
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
 
+RUST_SOURCES += src/csum_file.rs
 RUST_SOURCES += src/hash.rs
 RUST_SOURCES += src/lib.rs
 RUST_SOURCES += src/loose.rs
diff --git a/src/csum_file.rs b/src/csum_file.rs
new file mode 100644
index 0000000000..7f2c6c4fcb
--- /dev/null
+++ b/src/csum_file.rs
@@ -0,0 +1,81 @@
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation: version 2 of the License, dated June 1991.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <https://www.gnu.org/licenses/>.
+
+use crate::hash::{HashAlgorithm, GIT_MAX_RAWSZ};
+use std::ffi::CStr;
+use std::io::{self, Write};
+use std::os::raw::c_void;
+
+/// A writer that can write files identified by their hash or containing a trailing hash.
+pub struct HashFile {
+    ptr: *mut c_void,
+    algo: HashAlgorithm,
+}
+
+impl HashFile {
+    /// Create a new HashFile.
+    ///
+    /// The hash used will be `algo`, its name should be in `name`, and an open file descriptor
+    /// pointing to that file should be in `fd`.
+    pub fn new(algo: HashAlgorithm, fd: i32, name: &CStr) -> HashFile {
+        HashFile {
+            ptr: unsafe { c::hashfd(algo.hash_algo_ptr(), fd, name.as_ptr()) },
+            algo,
+        }
+    }
+
+    /// Finalize this HashFile instance.
+    ///
+    /// Returns the hash computed over the data.
+    pub fn finalize(self, component: u32, flags: u32) -> Vec<u8> {
+        let mut result = vec![0u8; GIT_MAX_RAWSZ];
+        unsafe { c::finalize_hashfile(self.ptr, result.as_mut_ptr(), component, flags) };
+        result.truncate(self.algo.raw_len());
+        result
+    }
+}
+
+impl Write for HashFile {
+    fn write(&mut self, data: &[u8]) -> io::Result<usize> {
+        for chunk in data.chunks(u32::MAX as usize) {
+            unsafe {
+                c::hashwrite(
+                    self.ptr,
+                    chunk.as_ptr() as *const c_void,
+                    chunk.len() as u32,
+                )
+            };
+        }
+        Ok(data.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        unsafe { c::hashflush(self.ptr) };
+        Ok(())
+    }
+}
+
+pub mod c {
+    use std::os::raw::{c_char, c_int, c_void};
+
+    extern "C" {
+        pub fn hashfd(algop: *const c_void, fd: i32, name: *const c_char) -> *mut c_void;
+        pub fn hashwrite(f: *mut c_void, data: *const c_void, len: u32);
+        pub fn hashflush(f: *mut c_void);
+        pub fn finalize_hashfile(
+            f: *mut c_void,
+            data: *mut u8,
+            component: u32,
+            flags: u32,
+        ) -> c_int;
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 442f9433dc..0c598298b1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
+pub mod csum_file;
 pub mod hash;
 pub mod loose;
 pub mod varint;
diff --git a/src/meson.build b/src/meson.build
index 1eea068519..45739957b4 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -1,4 +1,5 @@
 libgit_rs_sources = [
+  'csum_file.rs',
   'hash.rs',
   'lib.rs',
   'loose.rs',