1 files changed, 170 insertions, 0 deletions
diff --git a/src/diagnostics/error_document.rs b/src/diagnostics/error_document.rs
new file mode 100644
index 0000000..d3aab03
--- /dev/null
+++ b/src/diagnostics/error_document.rs
@@ -0,0 +1,170 @@
+//! Recognize top-level JSON error markers without buffering response values.
+//! This classifies provider error documents; it does not validate arbitrary JSON.
+
+#[derive(Default)]
+pub(super) struct ErrorDocument {
+    started: bool,
+    finished: bool,
+    depth: usize,
+    string: bool,
+    escaped: bool,
+    expect_key: bool,
+    key: Option<Vec<u8>>,
+    error_key: bool,
+    value: Option<ErrorValue>,
+    detected: bool,
+}
+
+enum ErrorValue {
+    Start,
+    String,
+    Container(u8),
+    Literal(Vec<u8>),
+}
+
+impl ErrorDocument {
+    pub fn feed(&mut self, bytes: &[u8]) {
+        for &byte in bytes {
+            if self.finished || self.detected {
+                break;
+            }
+            if !self.started {
+                // JSON producers sometimes include a UTF-8 BOM.
+                if byte.is_ascii_whitespace() || matches!(byte, 0xef | 0xbb | 0xbf) {
+                    continue;
+                }
+                self.started = true;
+                if byte != b'{' {
+                    self.finished = true;
+                    continue;
+                }
+                self.depth = 1;
+                self.expect_key = true;
+                continue;
+            }
+            self.inspect_value(byte);
+            if self.detected {
+                break;
+            }
+            if self.string {
+                if let Some(key) = &mut self.key {
+                    if key.len() < 64 {
+                        key.push(byte);
+                    } else {
+                        self.key = None;
+                    }
+                }
+                if self.escaped {
+                    self.escaped = false;
+                    continue;
+                }
+                if byte == b'\\' {
+                    self.escaped = true;
+                    continue;
+                }
+                if byte == b'"' {
+                    self.string = false;
+                    if let Some(key) = self.key.take() {
+                        self.error_key =
+                            serde_json::from_slice::<String>(&key)
+                                .ok()
+                                .is_some_and(|key| {
+                                    key.eq_ignore_ascii_case("error")
+                                        || key.eq_ignore_ascii_case("errors")
+                                });
+                        self.expect_key = false;
+                    }
+                }
+                continue;
+            }
+            match byte {
+                b'"' => {
+                    self.string = true;
+                    if self.depth == 1 && self.expect_key {
+                        self.key = Some(vec![b'"']);
+                    }
+                }
+                b'{' | b'[' => self.depth = self.depth.saturating_add(1),
+                b'}' | b']' => {
+                    self.depth = self.depth.saturating_sub(1);
+                    if self.depth == 0 {
+                        self.finished = true;
+                    }
+                }
+                b':' if self.depth == 1 && self.error_key => {
+                    self.value = Some(ErrorValue::Start);
+                    self.error_key = false;
+                }
+                b',' if self.depth == 1 => {
+                    self.expect_key = true;
+                    self.error_key = false;
+                }
+                _ => {}
+            }
+        }
+    }
+
+    fn inspect_value(&mut self, byte: u8) {
+        let Some(value) = self.value.take() else {
+            return;
+        };
+        self.value = match value {
+            ErrorValue::Start if byte.is_ascii_whitespace() => Some(ErrorValue::Start),
+            ErrorValue::Start => match byte {
+                b'"' => Some(ErrorValue::String),
+                b'[' => Some(ErrorValue::Container(b']')),
+                b'{' => Some(ErrorValue::Container(b'}')),
+                b'n' | b'f' | b'0' | b'-' => Some(ErrorValue::Literal(vec![byte])),
+                _ => {
+                    self.detected = true;
+                    None
+                }
+            },
+            ErrorValue::String => {
+                self.detected = byte != b'"';
+                None
+            }
+            ErrorValue::Container(end) if byte.is_ascii_whitespace() => {
+                Some(ErrorValue::Container(end))
+            }
+            ErrorValue::Container(end) => {
+                self.detected = byte != end;
+                None
+            }
+            ErrorValue::Literal(mut bytes) => {
+                if byte.is_ascii_whitespace() || matches!(byte, b',' | b'}') {
+                    self.detected = literal_is_error(&bytes);
+                    None
+                } else if bytes.len() < 64 {
+                    bytes.push(byte);
+                    Some(ErrorValue::Literal(bytes))
+                } else {
+                    self.detected = true;
+                    None
+                }
+            }
+        };
+    }
+
+    pub fn is_error(&self) -> bool {
+        self.detected
+            || match &self.value {
+                Some(ErrorValue::Literal(bytes)) => literal_is_error(bytes),
+                // An incomplete value of a declared error field is not a successful download.
+                Some(_) => true,
+                None => false,
+            }
+    }
+
+    pub fn confirmed_error(&self) -> bool {
+        self.detected
+    }
+}
+
+fn literal_is_error(bytes: &[u8]) -> bool {
+    match serde_json::from_slice::<serde_json::Value>(bytes) {
+        Ok(serde_json::Value::Null | serde_json::Value::Bool(false)) => false,
+        Ok(serde_json::Value::Number(number)) => number.as_f64() != Some(0.0),
+        _ => true,
+    }
+}