libgit: add higher-level libgit crate

The C functions exported by libgit-sys do not provide an idiomatic Rust
interface. To make it easier to use these functions via Rust, add a
higher-level "libgit" crate, that wraps the lower-level configset API
with an interface that is more Rust-y.

This combination of $X and $X-sys crates is a common pattern for FFI in
Rust, as documented in "The Cargo Book" [1].

[1] https://doc.rust-lang.org/cargo/reference/build-scripts.html#-sys-packages

Co-authored-by: Josh Steadmon <[email protected]>
Signed-off-by: Josh Steadmon <[email protected]>
Signed-off-by: Calvin Wan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

.gitignore

@@ -250,4 +250,5 @@ Release/
 /git.VC.db
 *.dSYM
 /contrib/buildsystems/out
+/contrib/libgit-rs/target
 /contrib/libgit-sys/target

Makefile

@@ -417,7 +417,7 @@ include shared.mak
 # programs in oss-fuzz/.
 #
 # Define INCLUDE_LIBGIT_RS if you want `make all` and `make test` to build and
-# test the Rust crate in contrib/libgit-sys.
+# test the Rust crates in contrib/libgit-sys and contrib/libgit-rs.
 #
 # === Optional library: libintl ===
 #
@@ -3741,7 +3741,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(htmldocs).tar.gz $(manpages).tar.gz
 	$(MAKE) -C Documentation/ clean
 	$(RM) Documentation/GIT-EXCLUDED-PROGRAMS
-	$(RM) -r contrib/libgit-sys/target
+	$(RM) -r contrib/libgit-sys/target contrib/libgit-rs/target
 	$(RM) contrib/libgit-sys/partial_symbol_export.o
 	$(RM) contrib/libgit-sys/hidden_symbol_export.o
 	$(RM) contrib/libgit-sys/libgitpub.a
@@ -3907,14 +3907,19 @@ build-unit-tests: $(UNIT_TEST_PROGS) $(CLAR_TEST_PROG)
 unit-tests: $(UNIT_TEST_PROGS) $(CLAR_TEST_PROG) t/helper/test-tool$X
 	$(MAKE) -C t/ unit-tests
 
-.PHONY: libgit-sys
+.PHONY: libgit-sys libgit-rs
 libgit-sys:
 	$(QUIET)(\
 		cd contrib/libgit-sys && \
 		cargo build \
 	)
+libgit-rs:
+	$(QUIET)(\
+		cd contrib/libgit-rs && \
+		cargo build \
+	)
 ifdef INCLUDE_LIBGIT_RS
-all:: libgit-sys
+all:: libgit-sys libgit-rs
 endif
 
 contrib/libgit-sys/partial_symbol_export.o: contrib/libgit-sys/public_symbol_export.o libgit.a reftable/libreftable.a xdiff/lib.a

contrib/libgit-rs/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "libgit"
+version = "0.1.0"
+edition = "2021"
+build = "build.rs"
+rust-version = "1.63" # TODO: Once we hit 1.84 or newer, we may want to remove Cargo.lock from
+                      # version control. See https://lore.kernel.org/git/[email protected]/
+
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+libgit-sys = { version = "0.1.0", path = "../libgit-sys" }
+
+[build-dependencies]
+autocfg = "1.4.0"

contrib/libgit-rs/README.md

@@ -0,0 +1,13 @@
+# libgit-rs
+
+Proof-of-concept Git bindings for Rust.
+
+```toml
+[dependencies]
+libgit = "0.1.0"
+```
+
+## Rust version requirements
+
+libgit-rs should support Rust versions at least as old as the version included
+in Debian stable (currently 1.63).

contrib/libgit-rs/build.rs

@@ -0,0 +1,4 @@
+pub fn main() {
+    let ac = autocfg::new();
+    ac.emit_has_path("std::ffi::c_char");
+}

contrib/libgit-rs/src/config.rs

@@ -0,0 +1,106 @@
+use std::ffi::{c_void, CStr, CString};
+use std::path::Path;
+
+#[cfg(has_std__ffi__c_char)]
+use std::ffi::{c_char, c_int};
+
+#[cfg(not(has_std__ffi__c_char))]
+#[allow(non_camel_case_types)]
+type c_char = i8;
+
+#[cfg(not(has_std__ffi__c_char))]
+#[allow(non_camel_case_types)]
+type c_int = i32;
+
+use libgit_sys::*;
+
+/// A ConfigSet is an in-memory cache for config-like files such as `.gitmodules` or `.gitconfig`.
+/// It does not support all config directives; notably, it will not process `include` or
+/// `includeIf` directives (but it will store them so that callers can choose whether and how to
+/// handle them).
+pub struct ConfigSet(*mut libgit_config_set);
+impl ConfigSet {
+    /// Allocate a new ConfigSet
+    pub fn new() -> Self {
+        unsafe { ConfigSet(libgit_configset_alloc()) }
+    }
+
+    /// Load the given files into the ConfigSet; conflicting directives in later files will
+    /// override those given in earlier files.
+    pub fn add_files(&mut self, files: &[&Path]) {
+        for file in files {
+            let pstr = file.to_str().expect("Invalid UTF-8");
+            let rs = CString::new(pstr).expect("Couldn't convert to CString");
+            unsafe {
+                libgit_configset_add_file(self.0, rs.as_ptr());
+            }
+        }
+    }
+
+    /// Load the value for the given key and attempt to parse it as an i32. Dies with a fatal error
+    /// if the value cannot be parsed. Returns None if the key is not present.
+    pub fn get_int(&mut self, key: &str) -> Option<i32> {
+        let key = CString::new(key).expect("Couldn't convert to CString");
+        let mut val: c_int = 0;
+        unsafe {
+            if libgit_configset_get_int(self.0, key.as_ptr(), &mut val as *mut c_int) != 0 {
+                return None;
+            }
+        }
+
+        Some(val.into())
+    }
+
+    /// Clones the value for the given key. Dies with a fatal error if the value cannot be
+    /// converted to a String. Returns None if the key is not present.
+    pub fn get_string(&mut self, key: &str) -> Option<String> {
+        let key = CString::new(key).expect("Couldn't convert key to CString");
+        let mut val: *mut c_char = std::ptr::null_mut();
+        unsafe {
+            if libgit_configset_get_string(self.0, key.as_ptr(), &mut val as *mut *mut c_char) != 0
+            {
+                return None;
+            }
+            let borrowed_str = CStr::from_ptr(val);
+            let owned_str =
+                String::from(borrowed_str.to_str().expect("Couldn't convert val to str"));
+            free(val as *mut c_void); // Free the xstrdup()ed pointer from the C side
+            Some(owned_str)
+        }
+    }
+}
+
+impl Default for ConfigSet {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Drop for ConfigSet {
+    fn drop(&mut self) {
+        unsafe {
+            libgit_configset_free(self.0);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn load_configs_via_configset() {
+        let mut cs = ConfigSet::new();
+        cs.add_files(&[
+            Path::new("testdata/config1"),
+            Path::new("testdata/config2"),
+            Path::new("testdata/config3"),
+        ]);
+        // ConfigSet retrieves correct value
+        assert_eq!(cs.get_int("trace2.eventTarget"), Some(1));
+        // ConfigSet respects last config value set
+        assert_eq!(cs.get_int("trace2.eventNesting"), Some(3));
+        // ConfigSet returns None for missing key
+        assert_eq!(cs.get_string("foo.bar"), None);
+    }
+}

contrib/libgit-rs/src/lib.rs

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

contrib/libgit-rs/testdata/config1

@@ -0,0 +1,2 @@
+[trace2]
+	eventNesting = 1

contrib/libgit-rs/testdata/config2

@@ -0,0 +1,2 @@
+[trace2]
+	eventTarget = 1

contrib/libgit-rs/testdata/config3

@@ -0,0 +1,2 @@
+[trace2]
+	eventNesting = 3

contrib/libgit-sys/src/lib.rs

@@ -1,3 +1,5 @@
+use std::ffi::c_void;
+
 #[cfg(has_std__ffi__c_char)]
 use std::ffi::{c_char, c_int};
 
@@ -19,6 +21,8 @@ pub struct libgit_config_set {
 }
 
 extern "C" {
+    pub fn free(ptr: *mut c_void);
+
     pub fn libgit_user_agent() -> *const c_char;
     pub fn libgit_user_agent_sanitized() -> *const c_char;
 

t/Makefile

@@ -178,12 +178,17 @@ perf:
 .PHONY: pre-clean $(T) aggregate-results clean valgrind perf \
 	check-chainlint clean-chainlint test-chainlint $(UNIT_TESTS)
 
-.PHONY: libgit-sys-test
+.PHONY: libgit-sys-test libgit-rs-test
 libgit-sys-test:
 	$(QUIET)(\
 		cd ../contrib/libgit-sys && \
 		cargo test \
 	)
+libgit-rs-test:
+	$(QUIET)(\
+		cd ../contrib/libgit-rs && \
+		cargo test \
+	)
 ifdef INCLUDE_LIBGIT_RS
-all:: libgit-sys-test
+all:: libgit-sys-test libgit-rs-test
 endif