Support a BKT_TMPDIR environment variable in addition to the --cache-…

…dir flag.

README.md

@@ -132,22 +132,27 @@ $ bkt --scope=foo -- date +%s.%N
 <a name="cache_dir"></a>
 ### Changing the Cache Directory
 
-By default, cached data is stored under `/tmp` or a similar temporary directory,
-this can be customized via the `--cache-dir` flag. Note that the choice of
-directory can affect `bkt`'s performance; if the cache is stored under a
-[`tmpfs`](https://en.wikipedia.org/wiki/Tmpfs) or solid-state partition it will
-be significantly faster than caching to a spinning disk.
+By default, cached data is stored under `/tmp` or a similar temporary directory;
+this can be customized via the `--cache-dir` flag, or by setting the
+`BKT_TMPDIR` environment variable. If both `BKT_TMPDIR` and `--cache-dir` are
+used the flag `--cache-dir` will take priority.
+
+Note that the choice of directory can affect `bkt`'s performance: if the cache
+is stored under a [`tmpfs`](https://en.wikipedia.org/wiki/Tmpfs) or solid-state
+partition it will be significantly faster than caching to a spinning disk.
 
 ## Security and Privacy
 
 The default cache directory is potentially world-readable. On Unix the cache
 directory is created with `700` permissions, meaning only the current user can
 access it, but this is not foolproof.
 
-You can customize the `--cache-dir` to a location you trust such as `~/.bkt`,
-but note that your home directory may be slower than the `TMPDIR` (see 
-[above](#cache_dir)). In general, if you are not the only user of your system
-it's wise to configure your `TMPDIR` to a location only you can access.
+You can customize the cache directory (see [above](#cache_dir)) to a location
+you trust such as `~/.bkt`, but note that your home directory may be slower than
+the temporary directory selected by default.
+
+In general, if you are not the only user of your system it's wise to configure
+your `TMPDIR` to a location only you can access.
 
 ## Patterns and Tips
 

src/lib.rs

@@ -706,13 +706,17 @@ pub struct Bkt {
 }
 
 impl Bkt {
+    fn temp_dir() -> PathBuf {
+        std::env::var_os("BKT_TMPDIR").map(PathBuf::from).unwrap_or_else(std::env::temp_dir)
+    }
+
     /// Creates a new Bkt instance using the [`std::env::temp_dir`] as the cache location.
     ///
     /// # Errors
     ///
     /// If preparing the tmp cache directory fails.
     pub fn in_tmp() -> Result<Self> {
-        Bkt::create(std::env::temp_dir())
+        Bkt::create(Bkt::temp_dir())
     }
 
     /// Creates a new Bkt instance.

tests/cli.rs

@@ -29,7 +29,7 @@ fn bkt<P: AsRef<Path>>(cache_dir: P) -> Command {
     }
     assert!(path.exists(), "Could not find bkt binary in {:?}", dir);
     let mut bkt = Command::new(&path);
-    bkt.arg(format!("--cache-dir={}", cache_dir.as_ref().display()));
+    bkt.env("BKT_TMPDIR", cache_dir.as_ref().as_os_str());
     bkt
 }
 
@@ -191,11 +191,11 @@ fn respects_cache_dir() {
     let file = dir.path("file");
     let args = vec!("--", "bash", "-c", COUNT_INVOCATIONS, "arg0", file.to_str().unwrap());
 
-    let first_call = succeed(bkt(dir.path("cache")).args(&args));
+    let first_call = succeed(bkt(dir.path("cache")).arg(format!("--cache-dir={}", dir.path("cache").display())).args(&args));
     assert_eq!(first_call, "1");
-    assert_eq!(first_call, succeed(bkt(dir.path("cache")).args(&args)));
+    assert_eq!(first_call, succeed(bkt(dir.path("cache")).arg(format!("--cache-dir={}", dir.path("cache").display())).args(&args)));
 
-    let diff_cache = succeed(bkt(dir.path("new-cache")).args(&args));
+    let diff_cache = succeed(bkt(dir.path("cache")).arg(format!("--cache-dir={}", dir.path("new-cache").display())).args(&args));
     assert_eq!(diff_cache, "2");
 }
 
@@ -368,14 +368,14 @@ fn force() {
     let args = vec!("--", "bash", "-c", COUNT_INVOCATIONS, "arg0", file.to_str().unwrap());
     let args_force = join(vec!("--force"), &args);
 
-    let output = succeed(bkt(&dir.path("cache")).args(&args));
+    let output = succeed(bkt(dir.path("cache")).args(&args));
     assert_eq!(output, "1");
-    let output = succeed(bkt(&dir.path("cache")).args(&args));
+    let output = succeed(bkt(dir.path("cache")).args(&args));
     assert_eq!(output, "1");
 
-    let output = succeed(bkt(&dir.path("cache")).args(&args_force));
+    let output = succeed(bkt(dir.path("cache")).args(&args_force));
     assert_eq!(output, "2");
-    let output = succeed(bkt(&dir.path("cache")).args(&args));
+    let output = succeed(bkt(dir.path("cache")).args(&args));
     assert_eq!(output, "2");
 }