feat: add SCCACHE_FILE_CLONE_COMPRESS for post-write compression

When SCCACHE_FILE_CLONE=true, users can set SCCACHE_FILE_CLONE_COMPRESS
to a command that will be spawned on each cache entry directory after
writing. The entry directory path is appended to the command.

Example: SCCACHE_FILE_CLONE_COMPRESS="applesauce compress -c lzfse"
applies APFS transparent compression on macOS, reducing cache disk
usage while keeping files transparently readable and reflink-capable.

The command runs in the background and failures are silently ignored.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mmastrac

docs/Configuration.md

@@ -224,6 +224,8 @@ export SCCACHE_MULTILEVEL_WRITE_POLICY="all"
 * `SCCACHE_CACHE_SIZE` maximum size of the local on disk cache i.e. `2G` - default is 10G
 * `SCCACHE_DIRECT` enable/disable preprocessor caching (see [the local doc](Local.md))
 * `SCCACHE_LOCAL_RW_MODE` the mode that the cache will operate in (`READ_ONLY` or `READ_WRITE`)
+* `SCCACHE_FILE_CLONE` enable reflink-based uncompressed disk cache (see [FileClone docs](FileClone.md))
+* `SCCACHE_FILE_CLONE_COMPRESS` command to run on each cache entry directory after writing (e.g., `applesauce compress -c lzfse`). The entry directory path is appended to the command. Only used when `SCCACHE_FILE_CLONE=true`.
 
 #### s3 compatible
 

src/cache/disk.rs

@@ -139,6 +139,26 @@ fn write_uncompressed_entry(cache_root: &Path, key_dir: &Path, entry: CacheWrite
         }
     }
 
+    // If SCCACHE_FILE_CLONE_COMPRESS is set, spawn the command with the entry
+    // directory appended. This allows users to apply transparent compression
+    // (e.g., APFS compression via `applesauce compress -c lzfse`).
+    if let Ok(cmd) = std::env::var("SCCACHE_FILE_CLONE_COMPRESS") {
+        if let Some(parts) = shlex::split(&cmd) {
+            if let Some((program, args)) = parts.split_first() {
+                let mut command = std::process::Command::new(program);
+                command
+                    .args(args)
+                    .arg(&entry_dir)
+                    .stdout(std::process::Stdio::null())
+                    .stderr(std::process::Stdio::null());
+                match command.spawn() {
+                    Ok(_) => trace!("Spawned file_clone compress: {} {:?}", cmd, entry_dir),
+                    Err(e) => trace!("Failed to spawn file_clone compress: {}", e),
+                }
+            }
+        }
+    }
+
     Ok(())
 }