add --check mode to wkg get

which compares the retrieved contents to an existing file, and exits with
0 if no difference or 1 if there is a difference.

Author: pchickey

crates/wkg/src/main.rs

@@ -199,6 +199,13 @@ struct GetArgs {
     #[arg(long, value_enum, default_value = "auto")]
     format: Format,
 
+    /// Check that the got package matches the existing file at the output
+    /// path. Output path will not be modified. Program exits with codes
+    /// simmilar to diff(1): exits with 1 if there were differences, and 0
+    /// means no differences.
+    #[arg(long)]
+    check: bool,
+
     /// Overwrite any existing output file.
     #[arg(long)]
     overwrite: bool,
@@ -269,6 +276,11 @@ enum Format {
 
 impl GetArgs {
     pub async fn run(self) -> anyhow::Result<()> {
+        ensure!(
+            !(self.overwrite && self.check),
+            "Not allowed to specify both --check and --overwrite"
+        );
+
         let PackageSpec { package, version } = self.package_spec;
         let mut config = self.common.load_config().await?;
         if let Some(registry) = self.registry_args.registry.clone() {
@@ -371,21 +383,34 @@ impl GetArgs {
         } else {
             self.output
         };
-        ensure!(
-            self.overwrite || !output_path.exists(),
-            "{output_path:?} already exists; you can use '--overwrite' to overwrite it"
-        );
 
-        if let Some(wit) = wit {
-            std::fs::write(&output_path, wit)
-                .with_context(|| format!("Failed to write WIT to {output_path:?}"))?
+        if self.check {
+            let existing = std::fs::read(&output_path)
+                .with_context(|| format!("Failed to read {output_path:?}"))?;
+            let latest = if let Some(wit) = wit {
+                wit.into_bytes()
+            } else {
+                std::fs::read(&tmp_path).with_context(|| format!("Failed to read {tmp_path:?}"))?
+            };
+            if existing != latest {
+                anyhow::bail!("Differences between retrieved and {output_path:?}");
+            }
         } else {
-            tmp_path
-                .persist(&output_path)
-                .with_context(|| format!("Failed to persist WASM to {output_path:?}"))?
-        }
-        println!("Wrote '{}'", output_path.display());
+            ensure!(
+                self.overwrite || !output_path.exists(),
+                "{output_path:?} already exists; you can use '--overwrite' to overwrite it"
+            );
 
+            if let Some(wit) = wit {
+                std::fs::write(&output_path, wit)
+                    .with_context(|| format!("Failed to write WIT to {output_path:?}"))?
+            } else {
+                tmp_path
+                    .persist(&output_path)
+                    .with_context(|| format!("Failed to persist WASM to {output_path:?}"))?
+            }
+            println!("Wrote '{}'", output_path.display());
+        }
         Ok(())
     }
 }

crates/wkg/tests/e2e.rs

@@ -87,3 +87,46 @@ async fn build_and_publish_with_metadata() {
         "Name should match"
     );
 }
+
+#[tokio::test]
+pub async fn check() {
+    let fixture = common::load_fixture("wasi-http").await;
+    let output = fixture.temp_dir.path().join("out");
+
+    let get = fixture
+        .command()
+        .arg("get")
+        .arg("wasi:http")
+        .arg("--output")
+        .arg(&output)
+        .status()
+        .await
+        .unwrap();
+    assert!(get.success());
+
+    let check_same = fixture
+        .command()
+        .arg("get")
+        .arg("--check")
+        .arg("wasi:http")
+        .arg("--output")
+        .arg(&output)
+        .status()
+        .await
+        .unwrap();
+    assert!(check_same.success());
+
+    std::fs::write(&output, vec![1, 2, 3, 4]).expect("overwrite output with bogus contents");
+
+    let check_diff = fixture
+        .command()
+        .arg("get")
+        .arg("--check")
+        .arg("wasi:http")
+        .arg("--output")
+        .arg(output)
+        .status()
+        .await
+        .unwrap();
+    assert!(!check_diff.success());
+}