Add a configuration option to fetch with git-the-CLI

Currently Cargo always uses `libgit2` to perform all fetches of git
repositories, but sometimes this is not sufficient. The `libgit2` library
doesn't support all authentication schemes that `git` does and it isn't always
quite at feature parity with `git` itself, especially in terms of network
configuration.

This commit adds a configuration option to Cargo for fetching git repositories
with the `git` CLI tool rather than the internal `libgit2`. While this exposes
us to changes over time in the `git` CLI it's hopefully a very targeted use case
that doesn't change much. The internal `libgit2` library should be sufficient
for all other forms of git repository management. (and using `git` for only
fetches shouldn't slow us down much)

The new configuration option in `.cargo/config` is:

    [net]
    git-fetch-with-cli = true

which can also be specified with `CARGO_NET_GIT_FETCH_WITH_CLI=true` via an
environment variable.

Closes #5903

Author: alexcrichton

src/cargo/sources/git/utils.rs

@@ -11,9 +11,10 @@ use serde::ser;
 use url::Url;
 
 use core::GitReference;
-use util::{internal, network, Config, Progress, ToUrl};
-use util::paths;
 use util::errors::{CargoError, CargoResult, CargoResultExt};
+use util::paths;
+use util::process_builder::process;
+use util::{internal, network, Config, Progress, ToUrl};
 
 #[derive(PartialEq, Clone, Debug)]
 pub struct GitRevision(git2::Oid);
@@ -691,6 +692,18 @@ pub fn fetch(
     // request we're about to issue.
     maybe_gc_repo(repo)?;
 
+    // Unfortuantely `libgit2` is notably lacking in the realm of authentication
+    // when compared to the `git` command line. As a result, allow an escape
+    // hatch for users that would prefer to use `git`-the-CLI for fetching
+    // repositories instead of `libgit2`-the-library. This should make more
+    // flavors of authentication possible while also still giving us all the
+    // speed and portability of using `libgit2`.
+    if let Some(val) = config.get_bool("net.git-fetch-with-cli")? {
+        if val.val {
+            return fetch_with_cli(repo, url, refspec, config);
+        }
+    }
+
     debug!("doing a fetch for {}", url);
     let git_config = git2::Config::open_default()?;
     with_fetch_options(&git_config, url, config, &mut |mut opts| {
@@ -732,6 +745,24 @@ pub fn fetch(
     })
 }
 
+fn fetch_with_cli(
+    repo: &mut git2::Repository,
+    url: &Url,
+    refspec: &str,
+    config: &Config,
+) -> CargoResult<()> {
+    let mut cmd = process("git");
+    cmd.arg("fetch")
+        .arg("--tags") // fetch all tags
+        .arg("--quiet")
+        .arg(url.to_string())
+        .arg(refspec)
+        .cwd(repo.path());
+    config.shell().verbose(|s| s.status("Running", &cmd.to_string()))?;
+    cmd.exec()?;
+    Ok(())
+}
+
 /// Cargo has a bunch of long-lived git repositories in its global cache and
 /// some, like the index, are updated very frequently. Right now each update
 /// creates a new "pack file" inside the git database, and over time this can

src/doc/src/reference/config.md

@@ -113,6 +113,7 @@ color = 'auto'         # whether cargo colorizes output
 # Network configuration
 [net]
 retry = 2 # number of times a network call will automatically retried
+git-fetch-with-cli = false  # if `true` we'll use `git`-the-CLI to fetch git repos
 
 # Alias cargo commands. The first 3 aliases are built in. If your
 # command requires grouped whitespace use the list format.

tests/testsuite/git.rs

@@ -2748,3 +2748,53 @@ fn failed_submodule_checkout() {
     drop(TcpStream::connect(&addr));
     t.join().unwrap();
 }
+
+#[test]
+fn use_the_cli() {
+    let project = project();
+    let git_project = git::new("dep1", |project| {
+        project.file("Cargo.toml", &basic_manifest("dep1", "0.5.0"))
+            .file("src/lib.rs", "")
+    }).unwrap();
+
+    let project = project
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [project]
+                    name = "foo"
+                    version = "0.5.0"
+                    authors = []
+
+                    [dependencies]
+                    dep1 = {{ git = '{}' }}
+                "#,
+                git_project.url()
+            ),
+        )
+        .file("src/lib.rs", "")
+        .file(
+            ".cargo/config",
+            "
+                [net]
+                git-fetch-with-cli = true
+            "
+        )
+        .build();
+
+    let stderr = "\
+[UPDATING] git repository `[..]`
+[RUNNING] `git fetch [..]`
+[COMPILING] dep1 [..]
+[RUNNING] `rustc [..]`
+[COMPILING] foo [..]
+[RUNNING] `rustc [..]`
+[FINISHED] [..]
+";
+
+    assert_that(
+        project.cargo("build -v"),
+        execs().with_stderr(stderr)
+    );
+}