Auto merge of #6748 - alexcrichton:release-then-acquire, r=ehuss

Release a jobserver token while locking a file

This is a possible solution to #6747, but we'll ideally get some testing
in before landing!

Cargo.toml

@@ -39,7 +39,7 @@ hex = "0.3"
 home = "0.3"
 ignore = "0.4"
 lazy_static = "1.2.0"
-jobserver = "0.1.11"
+jobserver = "0.1.13"
 lazycell = "1.2.0"
 libc = "0.2"
 log = "0.4.6"

appveyor.yml

@@ -2,16 +2,12 @@ environment:
   matrix:
   - TARGET: x86_64-pc-windows-msvc
     OTHER_TARGET: i686-pc-windows-msvc
-  - TARGET: x86_64-pc-windows-msvc
-    MINIMAL_VERSIONS: true
-    CFG_DISABLE_CROSS_TESTS: 1
 
 install:
   - if NOT defined APPVEYOR_PULL_REQUEST_NUMBER if "%APPVEYOR_REPO_BRANCH%" == "master" appveyor exit
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
-  - if defined MINIMAL_VERSIONS rustup toolchain install 1.31.0
   - if defined OTHER_TARGET rustup target add %OTHER_TARGET%
   - rustc -V
   - cargo -V
@@ -22,8 +18,4 @@ clone_depth: 1
 build: false
 
 test_script:
-  # we don't have ci time to run the full `cargo test` with `minimal-versions` like
-  # - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +stable test
-  # so we just run `cargo check --tests` like
-  - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +1.31.0 check --tests --features=deny-warnings
-  - if NOT defined MINIMAL_VERSIONS cargo test --features=deny-warnings
+  - cargo test --features=deny-warnings

src/cargo/util/flock.rs

@@ -312,8 +312,27 @@ fn acquire(
     let msg = format!("waiting for file lock on {}", msg);
     config.shell().status_with_color("Blocking", &msg, Cyan)?;
 
-    block().chain_err(|| format!("failed to lock file: {}", path.display()))?;
-    return Ok(());
+    // We're about to block the current process and not really do anything
+    // productive for what could possibly be a very long time. We could be
+    // waiting, for example, on another Cargo to finish a download, finish an
+    // entire build, etc. Since we're not doing anything productive we're not
+    // making good use of our jobserver token, if we have one.
+    //
+    // This can typically come about if `cargo` is invoked from `make` (or some
+    // other jobserver-providing system). In this situation it's actually best
+    // if we release the token back to the original jobserver to let some other
+    // cpu-hungry work continue to make progress. After we're done blocking
+    // we'll block waiting to reacquire a token as we'll probably be doing cpu
+    // hungry work ourselves.
+    let jobserver = config.jobserver_from_env();
+    if let Some(server) = jobserver {
+        server.release_raw()?;
+    }
+    let result = block().chain_err(|| format!("failed to lock file: {}", path.display()));
+    if let Some(server) = jobserver {
+        server.acquire_raw()?;
+    }
+    return Ok(result?);
 
     #[cfg(all(target_os = "linux", not(target_env = "musl")))]
     fn is_on_nfs_mount(path: &Path) -> bool {