fix: write OpenAPI YAML atomically

Cargo.toml

@@ -40,12 +40,12 @@ tracing-subscriber = "0.3"
 uuid = { version = "1", features = ["v4"] }
 rmp-serde = "1"
 globset = "0.4"
+tempfile = "3"
 
 [dev-dependencies]
 proptest = "1"
 assert_cmd = "2"
 predicates = "3"
-tempfile = "3"
 pretty_assertions = "1"
 
 [package.metadata.docs.rs]

src/output.rs

@@ -1,4 +1,5 @@
 use std::fs;
+use std::io::Write;
 use std::path::Path;
 
 use crate::error::Error;
@@ -58,11 +59,28 @@ pub fn templates_to_yaml(templates: &[String]) -> Result<String, Error> {
 }
 
 /// Write a YAML string to a file, creating parent directories if needed.
+///
+/// Uses an atomic write strategy: content is first written to a temporary file
+/// in the same directory, then renamed into place. If the write fails (e.g.
+/// disk full, permission denied), the original target file is left unchanged.
 pub fn write_yaml(content: &str, path: &Path) -> Result<(), Error> {
-    if let Some(parent) = path.parent() {
-        fs::create_dir_all(parent)?;
-    }
-    fs::write(path, content)?;
+    let parent = path.parent().ok_or_else(|| {
+        Error::Io(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            "output path has no parent directory",
+        ))
+    })?;
+    // Handle empty parent (relative path like "spec.yaml" → parent is "")
+    let parent = if parent.as_os_str().is_empty() {
+        Path::new(".")
+    } else {
+        parent
+    };
+    fs::create_dir_all(parent)?;
+
+    let mut tmp = tempfile::NamedTempFile::new_in(parent)?;
+    tmp.write_all(content.as_bytes())?;
+    tmp.persist(path).map_err(|e| Error::Io(e.error))?;
     Ok(())
 }
 

tests/output_atomic.rs

@@ -0,0 +1,67 @@
+use std::path::Path;
+
+#[test]
+fn successful_write_creates_file() {
+    let dir = tempfile::TempDir::new().unwrap();
+    let output = dir.path().join("spec.yaml");
+    mitm2openapi::output::write_yaml("hello: world\n", &output).unwrap();
+    assert_eq!(std::fs::read_to_string(&output).unwrap(), "hello: world\n");
+}
+
+#[test]
+fn overwrite_existing_file() {
+    let dir = tempfile::TempDir::new().unwrap();
+    let output = dir.path().join("spec.yaml");
+    mitm2openapi::output::write_yaml("v1: yes\n", &output).unwrap();
+    mitm2openapi::output::write_yaml("v2: yes\n", &output).unwrap();
+    assert_eq!(std::fs::read_to_string(&output).unwrap(), "v2: yes\n");
+}
+
+#[test]
+fn creates_parent_directories() {
+    let dir = tempfile::TempDir::new().unwrap();
+    let output = dir.path().join("nested").join("dir").join("spec.yaml");
+    mitm2openapi::output::write_yaml("nested: yes\n", &output).unwrap();
+    assert_eq!(std::fs::read_to_string(&output).unwrap(), "nested: yes\n");
+}
+
+#[cfg(target_os = "linux")]
+#[test]
+fn partial_write_preserves_target() {
+    use std::os::unix::fs::PermissionsExt;
+
+    let dir = tempfile::TempDir::new().unwrap();
+    let output = dir.path().join("spec.yaml");
+
+    mitm2openapi::output::write_yaml("original: yes\n", &output).unwrap();
+    let original = std::fs::read_to_string(&output).unwrap();
+    assert_eq!(original, "original: yes\n");
+
+    let mut perms = std::fs::metadata(dir.path()).unwrap().permissions();
+    perms.set_mode(0o500);
+    std::fs::set_permissions(dir.path(), perms).unwrap();
+
+    let result = mitm2openapi::output::write_yaml("new: content\n", &output);
+    assert!(result.is_err(), "write to read-only dir should fail");
+
+    let mut perms = std::fs::metadata(dir.path()).unwrap().permissions();
+    perms.set_mode(0o755);
+    std::fs::set_permissions(dir.path(), perms).unwrap();
+
+    let after = std::fs::read_to_string(&output).unwrap();
+    assert_eq!(
+        after, original,
+        "original file must remain untouched on write failure"
+    );
+}
+
+#[cfg(unix)]
+#[test]
+fn write_to_nonexistent_parent_fails_gracefully() {
+    // /nonexistent at filesystem root requires privilege on Unix; on Windows a
+    // leading slash without a drive letter is interpreted relative to the CWD,
+    // which would succeed and invert the assertion. Gate to Unix.
+    let result =
+        mitm2openapi::output::write_yaml("test\n", Path::new("/nonexistent/dir/spec.yaml"));
+    assert!(result.is_err());
+}