Implement read-only local cache

docs/Configuration.md

@@ -100,6 +100,7 @@ configuration variables
 * `SCCACHE_DIR` local on disk artifact cache directory
 * `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`)
 
 #### s3 compatible
 

docs/Local.md

@@ -43,3 +43,11 @@ Configuration options and their default values:
 See where to write the config in [the configuration doc](Configuration.md).
 
 *Note that preprocessor caching is currently only implemented for GCC and Clang and when using local storage.*
+
+## Read-only cache mode
+
+By default, the local cache operates in read/write mode. The `SCCACHE_LOCAL_RW_MODE` environment variable can be set to `READ_ONLY` (or `READ_WRITE`) to modify this behavior.
+
+You can use read-only mode to prevent sccache from writing new cache items to the disk. This can be useful, for example, if you want to use items that have already been cached, but not add new ones to the cache. 
+
+Note that this feature is only effective if you already have items in your cache. Using this option on an empty cache will cause sccache to simply do nothing, just add overhead.

src/cache/cache.rs

@@ -16,7 +16,7 @@
 use crate::cache::azure::AzureBlobCache;
 use crate::cache::disk::DiskCache;
 #[cfg(feature = "gcs")]
-use crate::cache::gcs::{GCSCache, RWMode};
+use crate::cache::gcs::GCSCache;
 #[cfg(feature = "gha")]
 use crate::cache::gha::GHACache;
 #[cfg(feature = "memcached")]
@@ -112,8 +112,8 @@ impl fmt::Debug for Cache {
     }
 }
 
-/// CacheMode is used to repreent which mode we are using.
-#[derive(Debug)]
+/// CacheMode is used to represent which mode we are using.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum CacheMode {
     /// Only read cache from storage.
     ReadOnly,
@@ -575,17 +575,12 @@ pub fn storage_from_config(
             }) => {
                 debug!("Init gcs cache with bucket {bucket}, key_prefix {key_prefix}");
 
-                let gcs_read_write_mode = match rw_mode {
-                    config::GCSCacheRWMode::ReadOnly => RWMode::ReadOnly,
-                    config::GCSCacheRWMode::ReadWrite => RWMode::ReadWrite,
-                };
-
                 let storage = GCSCache::build(
                     bucket,
                     key_prefix,
                     cred_path.as_deref(),
                     service_account.as_deref(),
-                    gcs_read_write_mode,
+                    (*rw_mode).into(),
                     credential_url.as_deref(),
                 )
                 .map_err(|err| anyhow!("create gcs cache failed: {err:?}"))?;
@@ -660,12 +655,14 @@ pub fn storage_from_config(
 
     let (dir, size) = (&config.fallback_cache.dir, config.fallback_cache.size);
     let preprocessor_cache_mode_config = config.fallback_cache.preprocessor_cache_mode;
+    let rw_mode = config.fallback_cache.rw_mode.into();
     debug!("Init disk cache with dir {:?}, size {}", dir, size);
     Ok(Arc::new(DiskCache::new(
         dir,
         size,
         pool,
         preprocessor_cache_mode_config,
+        rw_mode,
     )))
 }
 

src/cache/disk.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use crate::cache::{Cache, CacheRead, CacheWrite, Storage};
+use crate::cache::{Cache, CacheMode, CacheRead, CacheWrite, Storage};
 use crate::compiler::PreprocessorCacheEntry;
 use crate::lru_disk_cache::LruDiskCache;
 use crate::lru_disk_cache::{Error as LruError, ReadSeek};
@@ -65,6 +65,7 @@ pub struct DiskCache {
     pool: tokio::runtime::Handle,
     preprocessor_cache_mode_config: PreprocessorCacheModeConfig,
     preprocessor_cache: Arc<Mutex<LazyDiskCache>>,
+    rw_mode: CacheMode,
 }
 
 impl DiskCache {
@@ -74,6 +75,7 @@ impl DiskCache {
         max_size: u64,
         pool: &tokio::runtime::Handle,
         preprocessor_cache_mode_config: PreprocessorCacheModeConfig,
+        rw_mode: CacheMode,
     ) -> DiskCache {
         DiskCache {
             lru: Arc::new(Mutex::new(LazyDiskCache::Uninit {
@@ -88,6 +90,7 @@ impl DiskCache {
                     .into_os_string(),
                 max_size,
             })),
+            rw_mode,
         }
     }
 }
@@ -130,6 +133,11 @@ impl Storage for DiskCache {
         // We should probably do this on a background thread if we're going to buffer
         // everything in memory...
         trace!("DiskCache::finish_put({})", key);
+
+        if self.rw_mode == CacheMode::ReadOnly {
+            return Err(anyhow!("Cannot write to a read-only cache"));
+        }
+
         let lru = self.lru.clone();
         let key = make_key_path(key);
 
@@ -143,6 +151,10 @@ impl Storage for DiskCache {
             .await?
     }
 
+    async fn check(&self) -> Result<CacheMode> {
+        Ok(self.rw_mode)
+    }
+
     fn location(&self) -> String {
         format!("Local disk: {:?}", self.lru.lock().unwrap().path())
     }
@@ -171,6 +183,10 @@ impl Storage for DiskCache {
         key: &str,
         preprocessor_cache_entry: PreprocessorCacheEntry,
     ) -> Result<()> {
+        if self.rw_mode == CacheMode::ReadOnly {
+            return Err(anyhow!("Cannot write to a read-only cache"));
+        }
+
         let key = normalize_key(key);
         let mut buf = vec![];
         preprocessor_cache_entry.serialize_to(&mut buf)?;

src/cache/gcs.rs

@@ -13,6 +13,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use crate::cache::CacheMode;
 use crate::errors::*;
 use opendal::Operator;
 use opendal::{layers::LoggingLayer, services::Gcs};
@@ -21,18 +22,10 @@ use reqwest::Client;
 use serde::Deserialize;
 use url::Url;
 
-#[derive(Copy, Clone)]
-pub enum RWMode {
-    ReadOnly,
-    ReadWrite,
-}
-
-impl RWMode {
-    fn to_scope(self) -> &'static str {
-        match self {
-            RWMode::ReadOnly => "https://www.googleapis.com/auth/devstorage.read_only",
-            RWMode::ReadWrite => "https://www.googleapis.com/auth/devstorage.read_write",
-        }
+fn rw_to_scope(mode: CacheMode) -> &'static str {
+    match mode {
+        CacheMode::ReadOnly => "https://www.googleapis.com/auth/devstorage.read_only",
+        CacheMode::ReadWrite => "https://www.googleapis.com/auth/devstorage.read_write",
     }
 }
 
@@ -46,13 +39,13 @@ impl GCSCache {
         key_prefix: &str,
         cred_path: Option<&str>,
         service_account: Option<&str>,
-        rw_mode: RWMode,
+        rw_mode: CacheMode,
         credential_url: Option<&str>,
     ) -> Result<Operator> {
         let mut builder = Gcs::default();
         builder.bucket(bucket);
         builder.root(key_prefix);
-        builder.scope(rw_mode.to_scope());
+        builder.scope(rw_to_scope(rw_mode));
 
         if let Some(service_account) = service_account {
             builder.service_account(service_account);
@@ -67,7 +60,7 @@ impl GCSCache {
                 .map_err(|err| anyhow!("gcs credential url is invalid: {err:?}"))?;
 
             builder.customed_token_loader(Box::new(TaskClusterTokenLoader {
-                scope: rw_mode.to_scope().to_string(),
+                scope: rw_to_scope(rw_mode).to_string(),
                 url: cred_url.to_string(),
             }));
         }

src/compiler/c.rs

@@ -340,42 +340,51 @@ where
                     let mut updated = false;
                     let hit = preprocessor_cache_entry
                         .lookup_result_digest(preprocessor_cache_mode_config, &mut updated);
+
+                    let mut update_failed = false;
                     if updated {
                         // Time macros have been found, we need to update
                         // the preprocessor cache entry. See [`PreprocessorCacheEntry::result_matches`].
                         debug!(
                             "Preprocessor cache updated because of time macros: {preprocessor_key}"
                         );
-                        storage.put_preprocessor_cache_entry(
+
+                        if let Err(e) = storage.put_preprocessor_cache_entry(
                             preprocessor_key,
                             preprocessor_cache_entry,
-                        )?;
+                        ) {
+                            debug!("Failed to update preprocessor cache: {}", e);
+                            update_failed = true;
+                        }
                     }
-                    if let Some(key) = hit {
-                        debug!("Preprocessor cache hit: {preprocessor_key}");
-                        // A compiler binary may be a symlink to another and
-                        // so has the same digest, but that means
-                        // the toolchain will not contain the correct path
-                        // to invoke the compiler! Add the compiler
-                        // executable path to try and prevent this
-                        let weak_toolchain_key =
-                            format!("{}-{}", executable.to_string_lossy(), executable_digest);
-                        return Ok(HashResult {
-                            key,
-                            compilation: Box::new(CCompilation {
-                                parsed_args: parsed_args.to_owned(),
-                                #[cfg(feature = "dist-client")]
-                                // TODO or is it never relevant since dist?
-                                preprocessed_input: vec![],
-                                executable: executable.to_owned(),
-                                compiler: compiler.to_owned(),
-                                cwd: cwd.to_owned(),
-                                env_vars: env_vars.to_owned(),
-                            }),
-                            weak_toolchain_key,
-                        });
-                    } else {
-                        debug!("Preprocessor cache miss: {preprocessor_key}");
+
+                    if !update_failed {
+                        if let Some(key) = hit {
+                            debug!("Preprocessor cache hit: {preprocessor_key}");
+                            // A compiler binary may be a symlink to another and
+                            // so has the same digest, but that means
+                            // the toolchain will not contain the correct path
+                            // to invoke the compiler! Add the compiler
+                            // executable path to try and prevent this
+                            let weak_toolchain_key =
+                                format!("{}-{}", executable.to_string_lossy(), executable_digest);
+                            return Ok(HashResult {
+                                key,
+                                compilation: Box::new(CCompilation {
+                                    parsed_args: parsed_args.to_owned(),
+                                    #[cfg(feature = "dist-client")]
+                                    // TODO or is it never relevant since dist?
+                                    preprocessed_input: vec![],
+                                    executable: executable.to_owned(),
+                                    compiler: compiler.to_owned(),
+                                    cwd: cwd.to_owned(),
+                                    env_vars: env_vars.to_owned(),
+                                }),
+                                weak_toolchain_key,
+                            });
+                        } else {
+                            debug!("Preprocessor cache miss: {preprocessor_key}");
+                        }
                     }
                 }
             }
@@ -491,8 +500,12 @@ where
                     .collect();
                 files.sort_unstable_by(|a, b| a.1.cmp(&b.1));
                 preprocessor_cache_entry.add_result(start_of_compilation, &key, files);
-                storage
-                    .put_preprocessor_cache_entry(&preprocessor_key, preprocessor_cache_entry)?;
+
+                if let Err(e) = storage
+                    .put_preprocessor_cache_entry(&preprocessor_key, preprocessor_cache_entry)
+                {
+                    debug!("Failed to update preprocessor cache: {}", e);
+                }
             }
         }
 

src/compiler/compiler.rs

@@ -1381,7 +1381,7 @@ where
 mod test {
     use super::*;
     use crate::cache::disk::DiskCache;
-    use crate::cache::{CacheRead, PreprocessorCacheModeConfig};
+    use crate::cache::{CacheMode, CacheRead, PreprocessorCacheModeConfig};
     use crate::mock_command::*;
     use crate::test::mock_storage::MockStorage;
     use crate::test::utils::*;
@@ -1824,6 +1824,7 @@ LLVM version: 6.0",
                 use_preprocessor_cache_mode: preprocessor_cache_mode,
                 ..Default::default()
             },
+            CacheMode::ReadWrite,
         );
         // Write a dummy input file so the preprocessor cache mode can work
         std::fs::write(f.tempdir.path().join("foo.c"), "whatever").unwrap();
@@ -1949,6 +1950,7 @@ LLVM version: 6.0",
                 use_preprocessor_cache_mode: preprocessor_cache_mode,
                 ..Default::default()
             },
+            CacheMode::ReadWrite,
         );
         // Write a dummy input file so the preprocessor cache mode can work
         std::fs::write(f.tempdir.path().join("foo.c"), "whatever").unwrap();
@@ -2236,6 +2238,7 @@ LLVM version: 6.0",
                 use_preprocessor_cache_mode: preprocessor_cache_mode,
                 ..Default::default()
             },
+            CacheMode::ReadWrite,
         );
         let storage = Arc::new(storage);
         // Write a dummy input file so the preprocessor cache mode can work
@@ -2361,6 +2364,7 @@ LLVM version: 6.0",
                 use_preprocessor_cache_mode: preprocessor_cache_mode,
                 ..Default::default()
             },
+            CacheMode::ReadWrite,
         );
         let storage = Arc::new(storage);
         // Pretend to be GCC.  Also inject a fake object file that the subsequent
@@ -2454,6 +2458,7 @@ LLVM version: 6.0",
                 use_preprocessor_cache_mode: preprocessor_cache_mode,
                 ..Default::default()
             },
+            CacheMode::ReadWrite,
         );
         let storage = Arc::new(storage);
         // Pretend to be GCC.