rust: implement a test balloon via the "varint" subsystem

Implement a trivial test balloon for our Rust build infrastructure by
reimplementing the "varint.c" subsystem in Rust. This subsystem is
chosen because it is trivial to convert and because it doesn't have any
dependencies to other components of Git.

If support for Rust is enabled, we stop compiling "varint.c" and instead
compile and use "src/varint.rs".

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1307,7 +1307,9 @@ LIB_OBJS += urlmatch.o
 LIB_OBJS += usage.o
 LIB_OBJS += userdiff.o
 LIB_OBJS += utf8.o
+ifndef WITH_RUST
 LIB_OBJS += varint.o
+endif
 LIB_OBJS += version.o
 LIB_OBJS += versioncmp.o
 LIB_OBJS += walker.o
@@ -1499,6 +1501,7 @@ CLAR_TEST_OBJS += $(UNIT_TEST_DIR)/unit-test.o
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
 
 RUST_SOURCES += src/lib.rs
+RUST_SOURCES += src/varint.rs
 
 GIT-VERSION-FILE: FORCE
 	@OLD=$$(cat $@ 2>/dev/null || :) && \

meson.build

@@ -522,7 +522,6 @@ libgit_sources = [
   'usage.c',
   'userdiff.c',
   'utf8.c',
-  'varint.c',
   'version.c',
   'versioncmp.c',
   'walker.c',
@@ -1716,6 +1715,10 @@ rust_option = get_option('rust').disable_auto_if(not rust_available)
 if rust_option.allowed()
   subdir('src')
   libgit_c_args += '-DWITH_RUST'
+else
+  libgit_sources += [
+    'varint.c',
+  ]
 endif
 
 libgit = declare_dependency(

src/lib.rs

@@ -0,0 +1 @@
+pub mod varint;

src/meson.build

@@ -1,5 +1,6 @@
 libgit_rs_sources = [
   'lib.rs',
+  'varint.rs',
 ]
 
 if meson.version().version_compare('>=1.5.0')

src/varint.rs

@@ -0,0 +1,95 @@
+use std::os::raw::c_int;
+use std::os::raw::c_uchar;
+
+#[no_mangle]
+pub unsafe extern "C" fn decode_varint(bufp: *mut *const c_uchar) -> usize {
+    let mut buf = *bufp;
+    let mut c = *buf;
+    let mut val = usize::from(c & 127);
+
+    buf = buf.add(1);
+
+    while (c & 128) != 0 {
+        val = val.wrapping_add(1);
+        if val == 0 || val.leading_zeros() < 7 {
+            return 0; // overflow
+        }
+
+        c = *buf;
+        buf = buf.add(1);
+
+        val = (val << 7) + usize::from(c & 127);
+    }
+
+    *bufp = buf;
+    val
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn encode_varint(value: usize, buf: *mut c_uchar) -> c_int {
+    let mut varint: [u8; 16] = [0; 16];
+    let mut pos = varint.len() - 1;
+
+    varint[pos] = (value & 127) as u8;
+
+    let mut value = value >> 7;
+    while value != 0 {
+        pos -= 1;
+        value -= 1;
+        varint[pos] = 128 | (value & 127) as u8;
+        value >>= 7;
+    }
+
+    if !buf.is_null() {
+        std::ptr::copy_nonoverlapping(varint.as_ptr().add(pos), buf, varint.len() - pos);
+    }
+
+    (varint.len() - pos) as c_int
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_decode_varint() {
+        unsafe {
+            assert_eq!(decode_varint(&mut [0x00].as_slice().as_ptr()), 0);
+            assert_eq!(decode_varint(&mut [0x01].as_slice().as_ptr()), 1);
+            assert_eq!(decode_varint(&mut [0x7f].as_slice().as_ptr()), 127);
+            assert_eq!(decode_varint(&mut [0x80, 0x00].as_slice().as_ptr()), 128);
+            assert_eq!(decode_varint(&mut [0x80, 0x01].as_slice().as_ptr()), 129);
+            assert_eq!(decode_varint(&mut [0x80, 0x7f].as_slice().as_ptr()), 255);
+
+            // Overflows are expected to return 0.
+            assert_eq!(decode_varint(&mut [0x88; 16].as_slice().as_ptr()), 0);
+        }
+    }
+
+    #[test]
+    fn test_encode_varint() {
+        unsafe {
+            let mut varint: [u8; 16] = [0; 16];
+
+            assert_eq!(encode_varint(0, std::ptr::null_mut()), 1);
+
+            assert_eq!(encode_varint(0, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [0; 16]);
+
+            assert_eq!(encode_varint(10, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [10, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(127, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [127, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(128, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(129, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(255, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 127, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+        }
+    }
+}