chore: only warn about metadata cache once (#4807)

Author: ruben-arts

Cargo.lock

@@ -20,6 +20,7 @@ fs-err = { workspace = true }
 futures = { workspace = true }
 itertools = { workspace = true }
 miette = { workspace = true }
+once_cell = { workspace = true }
 ordermap = { workspace = true }
 pathdiff = { workspace = true }
 pin-project-lite = { workspace = true }

crates/pixi_command_dispatcher/Cargo.toml

@@ -1,10 +1,5 @@
-use std::{
-    collections::{BTreeMap, BTreeSet},
-    hash::{Hash, Hasher},
-    path::PathBuf,
-};
-
 use miette::Diagnostic;
+use once_cell::sync::Lazy;
 use pathdiff::diff_paths;
 use pixi_build_discovery::{CommandSpec, EnabledProtocols};
 use pixi_build_frontend::Backend;
@@ -14,6 +9,12 @@ use pixi_record::{InputHash, PinnedSourceSpec};
 use pixi_spec::{SourceAnchor, SourceSpec};
 use rand::random;
 use rattler_conda_types::{ChannelConfig, ChannelUrl};
+use std::{
+    collections::{BTreeMap, BTreeSet, HashSet},
+    hash::{Hash, Hasher},
+    path::PathBuf,
+    sync::Mutex,
+};
 use thiserror::Error;
 use tracing::instrument;
 use xxhash_rust::xxh3::Xxh3;
@@ -29,6 +30,18 @@ use crate::{
 use pixi_build_discovery::BackendSpec;
 use pixi_build_frontend::BackendOverride;
 
+static WARNED_BACKENDS: Lazy<Mutex<HashSet<String>>> = Lazy::new(|| Mutex::new(HashSet::new()));
+
+fn warn_once_per_backend(backend_name: &str) {
+    let mut warned = WARNED_BACKENDS.lock().unwrap();
+    if warned.insert(backend_name.to_string()) {
+        tracing::warn!(
+            "metadata cache disabled for build backend '{}' (system/path-based backends always regenerate metadata)",
+            backend_name
+        );
+    }
+}
+
 /// Represents a request for metadata from a build backend for a particular
 /// source location. The result of this request is the metadata for that
 /// particular source.
@@ -145,10 +158,7 @@ impl BuildBackendMetadataSpec {
             let backend_name = match &discovered_backend.backend_spec {
                 BackendSpec::JsonRpc(spec) => &spec.name,
             };
-            tracing::warn!(
-                "metadata cache disabled for build backend '{}' (system/path-based backends always regenerate metadata)",
-                backend_name
-            );
+            warn_once_per_backend(backend_name);
         }
 
         // Instantiate the backend with the discovered information.