Fix busy-loop when proc-macro prints to stdout

src/lib.rs

@@ -56,7 +56,8 @@
 //!     .spawn()
 //!     .unwrap();
 //!
-//! for message in cargo_metadata::parse_messages(command.stdout.take().unwrap()) {
+//! let reader = std::io::BufReader::new(command.stdout.take().unwrap());
+//! for message in cargo_metadata::Message::parse_stream(reader) {
 //!     match message.unwrap() {
 //!         Message::CompilerMessage(msg) => {
 //!             println!("{:?}", msg);
@@ -96,8 +97,10 @@ pub use dependency::{Dependency, DependencyKind};
 use diagnostic::Diagnostic;
 pub use errors::{Error, Result};
 pub use messages::{
-    parse_messages, Artifact, ArtifactProfile, BuildFinished, BuildScript, CompilerMessage, Message,
+    Artifact, ArtifactProfile, BuildFinished, BuildScript, CompilerMessage, Message, MessageIter
 };
+#[allow(deprecated)]
+pub use messages::parse_messages;
 
 mod dependency;
 pub mod diagnostic;

src/messages.rs

@@ -1,7 +1,7 @@
 use super::{Diagnostic, PackageId, Target};
 use serde_json;
 use std::fmt;
-use std::io::Read;
+use std::io::{self, BufRead, Lines, Read};
 use std::path::PathBuf;
 
 /// Profile settings used to determine which compiler flags to use for a
@@ -110,23 +110,51 @@ pub enum Message {
     /// This is emitted at the end of the build as the last message.
     /// Added in Rust 1.44.
     BuildFinished(BuildFinished),
+    /// A line of text which isn't a cargo or compiler message.
+    /// Line separator is not included
+    TextLine(String),
     #[doc(hidden)]
     #[serde(other)]
     Unknown,
 }
 
+impl Message {
+    /// Creates an iterator of Message from a Read outputting a stream of JSON
+    /// messages. For usage information, look at the top-level documentation.
+    pub fn parse_stream<R: BufRead>(input: R) -> MessageIter<R> {
+        MessageIter {
+            lines: input.lines(),
+        }
+    }
+}
+
 impl fmt::Display for CompilerMessage {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{}", self.message)
     }
 }
 
+/// An iterator of Messages.
+pub struct MessageIter<R> {
+    lines: Lines<R>,
+}
+
+impl<R: BufRead> Iterator for MessageIter<R> {
+    type Item = io::Result<Message>;
+    fn next(&mut self) -> Option<Self::Item> {
+        let line = self.lines.next()?;
+        let message = line.map(|it| serde_json::from_str(&it).unwrap_or(Message::TextLine(it)));
+        Some(message)
+    }
+}
+
 /// An iterator of Message.
 type MessageIterator<R> =
     serde_json::StreamDeserializer<'static, serde_json::de::IoRead<R>, Message>;
 
 /// Creates an iterator of Message from a Read outputting a stream of JSON
 /// messages. For usage information, look at the top-level documentation.
+#[deprecated(note = "Use Message::parse_stream instead")]
 pub fn parse_messages<R: Read>(input: R) -> MessageIterator<R> {
     serde_json::Deserializer::from_reader(input).into_iter::<Message>()
 }

tests/test_samples.rs

@@ -474,3 +474,24 @@ fn current_dir() {
     let namedep = meta.packages.iter().find(|p| p.name == "namedep").unwrap();
     assert!(namedep.name.starts_with("namedep"));
 }
+
+#[test]
+fn parse_stream_is_robust() {
+    // Proc macros can print stuff to stdout, which naturally breaks JSON messages.
+    // Let's check that we don't die horribly in this case, and report an error.
+    let json_output = r##"{"reason":"compiler-artifact","package_id":"chatty 0.1.0 (path+file:///chatty-macro/chatty)","target":{"kind":["proc-macro"],"crate_types":["proc-macro"],"name":"chatty","src_path":"/chatty-macro/chatty/src/lib.rs","edition":"2018","doctest":true},"profile":{"opt_level":"0","debuginfo":2,"debug_assertions":true,"overflow_checks":true,"test":false},"features":[],"filenames":["/chatty-macro/target/debug/deps/libchatty-f2adcff24cdf3bb2.so"],"executable":null,"fresh":false}
+Evil proc macro was here!
+{"reason":"compiler-artifact","package_id":"chatty-macro 0.1.0 (path+file:///chatty-macro)","target":{"kind":["lib"],"crate_types":["lib"],"name":"chatty-macro","src_path":"/chatty-macro/src/lib.rs","edition":"2018","doctest":true},"profile":{"opt_level":"0","debuginfo":2,"debug_assertions":true,"overflow_checks":true,"test":false},"features":[],"filenames":["/chatty-macro/target/debug/libchatty_macro.rlib","/chatty-macro/target/debug/deps/libchatty_macro-cb5956ed52a11fb6.rmeta"],"executable":null,"fresh":false}
+"##;
+    let mut n_messages = 0;
+    let mut text = String::new();
+    for message in cargo_metadata::Message::parse_stream(json_output.as_bytes()) {
+        let message = message.unwrap();
+        match message {
+            cargo_metadata::Message::TextLine(line) => text = line,
+            _ => n_messages += 1,
+        }
+    }
+    assert_eq!(n_messages, 2);
+    assert_eq!(text, "Evil proc macro was here!");
+}