mailsis_utils/

mime.rs

//! MIME validation and raw header parsing.
//!
//! Incoming messages need their headers inspected at several points in the
//! pipeline, to check for a `MIME-Version` header per
//! [RFC 2045](https://www.rfc-editor.org/rfc/rfc2045), to extract
//! structured key-value pairs, or to split headers from the body.

use std::{collections::HashMap, error::Error};

/// Checks whether a raw email contains a `MIME-Version:` header,
/// indicating it is a valid MIME message per RFC 2045.
///
/// Only the header section (lines before the first blank line) is inspected.
/// A `MIME-Version:` header appearing in the content (after the blank
/// line separator) is not considered valid.
///
/// # Examples
///
/// A message with a `MIME-Version` header is valid:
///
/// ```rust
/// assert!(mailsis_utils::is_mime_valid(
///     "MIME-Version: 1.0\r\nContent-Type: text/plain\r\n\r\nBody"
/// ));
/// ```
///
/// A message without a `MIME-Version` header is not valid:
///
/// ```rust
/// assert!(!mailsis_utils::is_mime_valid("Subject: Hello\r\n\r\nBody"));
/// ```
///
/// `MIME-Version` in the body (after the blank line) does not count:
///
/// ```rust
/// assert!(!mailsis_utils::is_mime_valid(
///     "Subject: Hello\r\n\r\nMIME-Version: 1.0"
/// ));
/// ```
pub fn is_mime_valid(raw: &str) -> bool {
    for line in raw.lines() {
        if line.trim().is_empty() {
            break;
        }
        if line.trim().starts_with("MIME-Version:") {
            return true;
        }
    }
    false
}

/// Parses the headers of a MIME email, returning a hashmap of the headers.
///
/// The parsing is done by splitting the raw email on the first empty line,
/// and splitting each line on the colon, separating the key and value.
///
/// Aligned with the RFC 2045, the headers are case-insensitive.
///
/// # Examples
///
/// ```rust
/// let expected = [("From", "test@example.com"), ("To", "test@example.com")]
///     .into_iter()
///     .map(|(k, v)| (k.to_string(), v.to_string()))
///     .collect::<std::collections::HashMap<_, _>>();
/// let headers = mailsis_utils::parse_mime_headers("From: test@example.com\r\nTo: test@example.com\r\n").unwrap();
/// assert_eq!(headers, expected);
/// ```
///
/// ```rust
/// let expected = [("From", "test@example.com"), ("To", "test@example.com")]
///     .into_iter()
///     .map(|(k, v)| (k.to_string(), v.to_string()))
///     .collect::<std::collections::HashMap<_, _>>();
/// let headers = mailsis_utils::parse_mime_headers("From: test@example.com\r\nTo: test@example.com\r\n\r\nBody of the email").unwrap();
/// assert_eq!(headers, expected);
/// ```
pub fn parse_mime_headers(raw: &str) -> Result<HashMap<String, String>, Box<dyn Error>> {
    let mut headers = HashMap::new();
    for line in raw.lines() {
        if line.trim().is_empty() {
            break;
        }
        if let Some((key, value)) = line.split_once(':') {
            headers.insert(key.trim().to_string(), value.trim().to_string());
        }
    }
    Ok(headers)
}

/// Parses headers from a raw email, returning an ordered list of headers
/// and a reference to the content after the blank-line separator.
///
/// Headers are preserved in their original order with case-preserved keys
/// and trimmed values. This supports duplicate headers (e.g. `Received`).
///
/// # Examples
///
/// ```rust
/// let (headers, content) = mailsis_utils::parse_raw_headers(
///     "From: alice@example.com\r\nTo: bob@example.com\r\n\r\nHello!"
/// );
/// assert_eq!(headers.len(), 2);
/// assert_eq!(headers[0], ("From".to_string(), "alice@example.com".to_string()));
/// assert_eq!(content, "Hello!");
/// ```
pub fn parse_raw_headers(raw: &str) -> (Vec<(String, String)>, &str) {
    let mut headers = Vec::new();
    let mut pos = 0;

    for line in raw.lines() {
        let line_len = line.len();
        let end = pos + line_len;
        let consumed = if raw[end..].starts_with("\r\n") {
            end + 2
        } else if raw[end..].starts_with('\n') {
            end + 1
        } else {
            end
        };

        if line.trim().is_empty() {
            pos = consumed;
            break;
        }

        if let Some((key, value)) = line.split_once(':') {
            headers.push((key.trim().to_string(), value.trim().to_string()));
        } else {
            // Line is not a header (no colon) and not blank, treat as start of content
            break;
        }

        pos = consumed;
    }

    (headers, &raw[pos..])
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_is_mime_valid_with_mime_version() {
        assert!(is_mime_valid(
            "MIME-Version: 1.0\r\nContent-Type: text/plain\r\n\r\nBody"
        ));
    }

    #[test]
    fn test_is_mime_valid_without_mime_version() {
        assert!(!is_mime_valid("Subject: Hello\r\n\r\nBody"));
    }

    #[test]
    fn test_is_mime_valid_mime_in_body_only() {
        assert!(!is_mime_valid("Subject: Hello\r\n\r\nMIME-Version: 1.0"));
    }

    #[test]
    fn test_is_mime_valid_empty_input() {
        assert!(!is_mime_valid(""));
    }

    #[test]
    fn test_is_mime_valid_no_headers() {
        assert!(!is_mime_valid("Plain text without headers"));
    }

    #[test]
    fn test_is_mime_valid_mime_version_among_headers() {
        assert!(is_mime_valid(
            "From: a@b.com\r\nMIME-Version: 1.0\r\nTo: c@d.com\r\n\r\nBody"
        ));
    }

    #[test]
    fn test_parse_mime_headers_basic() {
        let headers =
            parse_mime_headers("From: test@example.com\r\nTo: test@example.com\r\n").unwrap();
        assert_eq!(headers.len(), 2);
        assert_eq!(headers["From"], "test@example.com");
        assert_eq!(headers["To"], "test@example.com");
    }

    #[test]
    fn test_parse_mime_headers_with_body() {
        let headers =
            parse_mime_headers("From: a@b.com\r\nSubject: Test\r\n\r\nBody content here").unwrap();
        assert_eq!(headers.len(), 2);
        assert_eq!(headers["Subject"], "Test");
        assert!(!headers.contains_key("Body content here"));
    }

    #[test]
    fn test_parse_mime_headers_empty_input() {
        let headers = parse_mime_headers("").unwrap();
        assert!(headers.is_empty());
    }

    #[test]
    fn test_parse_mime_headers_no_headers() {
        let headers = parse_mime_headers("\r\nBody only").unwrap();
        assert!(headers.is_empty());
    }

    #[test]
    fn test_parse_mime_headers_duplicate_keys() {
        let headers = parse_mime_headers("X-Custom: first\r\nX-Custom: second\r\n").unwrap();
        assert_eq!(headers.len(), 1);
        assert_eq!(headers["X-Custom"], "second");
    }

    #[test]
    fn test_parse_raw_headers_basic() {
        let (headers, content) =
            parse_raw_headers("From: alice@example.com\r\nTo: bob@example.com\r\n\r\nHello!");
        assert_eq!(headers.len(), 2);
        assert_eq!(
            headers[0],
            ("From".to_string(), "alice@example.com".to_string())
        );
        assert_eq!(
            headers[1],
            ("To".to_string(), "bob@example.com".to_string())
        );
        assert_eq!(content, "Hello!");
    }

    #[test]
    fn test_parse_raw_headers_preserves_order() {
        let (headers, _) = parse_raw_headers("Z-Last: z\r\nA-First: a\r\nM-Middle: m\r\n\r\nBody");
        assert_eq!(headers[0].0, "Z-Last");
        assert_eq!(headers[1].0, "A-First");
        assert_eq!(headers[2].0, "M-Middle");
    }

    #[test]
    fn test_parse_raw_headers_duplicate_headers() {
        let (headers, _) = parse_raw_headers("Received: first\r\nReceived: second\r\n\r\nBody");
        assert_eq!(headers.len(), 2);
        assert_eq!(headers[0].1, "first");
        assert_eq!(headers[1].1, "second");
    }

    #[test]
    fn test_parse_raw_headers_empty_input() {
        let (headers, content) = parse_raw_headers("");
        assert!(headers.is_empty());
        assert_eq!(content, "");
    }

    #[test]
    fn test_parse_raw_headers_no_headers() {
        let (headers, content) = parse_raw_headers("Plain text body");
        assert!(headers.is_empty());
        assert_eq!(content, "Plain text body");
    }

    #[test]
    fn test_parse_raw_headers_empty_body() {
        let (headers, content) = parse_raw_headers("Subject: Test\r\n\r\n");
        assert_eq!(headers.len(), 1);
        assert_eq!(content, "");
    }

    #[test]
    fn test_parse_raw_headers_lf_line_endings() {
        let (headers, content) = parse_raw_headers("From: a@b.com\nTo: c@d.com\n\nBody");
        assert_eq!(headers.len(), 2);
        assert_eq!(content, "Body");
    }

    #[test]
    fn test_parse_raw_headers_trims_values() {
        let (headers, _) = parse_raw_headers("Subject:  spaced value  \r\n\r\nBody");
        assert_eq!(headers[0].1, "spaced value");
    }
}