feat(host): CRLF in temporary EML files

According to [1], CRLF should be used in EML files regardless of
platform. Thunderbird also commits to this standard for better
interoperability [2].

And Vim/Neovim actually set fileformat=dos by default for EML files even
under Linux/macOS, hence sending CRLF to Thunderbird directly shouldn't
be an issue either.

[1] https://datatracker.ietf.org/doc/html/rfc2822
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=503271

src/model/messaging.rs

@@ -3,6 +3,7 @@ use serde::{Deserialize, Serialize};
 use std::io;
 
 use super::thunderbird::*;
+use crate::writeln_crlf;
 
 pub const MAX_BODY_LENGTH: usize = 768 * 1024;
 
@@ -51,21 +52,22 @@ impl Exchange {
     where
         W: io::Write,
     {
-        writeln!(w, "From: {}", self.compose_details.from.to_header_value()?)?;
+        writeln_crlf!(w, "From: {}", self.compose_details.from.to_header_value()?)?;
         Self::compose_recipient_list_to_eml(w, "To", &self.compose_details.to)?;
         Self::compose_recipient_list_to_eml(w, "Cc", &self.compose_details.cc)?;
         Self::compose_recipient_list_to_eml(w, "Bcc", &self.compose_details.bcc)?;
         Self::compose_recipient_list_to_eml(w, "Reply-To", &self.compose_details.reply_to)?;
-        writeln!(w, "Subject: {}", self.compose_details.subject)?;
-        writeln!(
+        writeln_crlf!(w, "Subject: {}", self.compose_details.subject)?;
+        writeln_crlf!(
             w,
             "{}: {}",
-            HEADER_SEND_ON_EXIT, self.configuration.send_on_exit
+            HEADER_SEND_ON_EXIT,
+            self.configuration.send_on_exit
         )?;
         if !self.configuration.suppress_help_headers {
             Self::write_help_headers(w)?;
         }
-        writeln!(w)?;
+        writeln_crlf!(w)?;
         write!(w, "{}", self.compose_details.get_body())?;
         Ok(())
     }
@@ -189,14 +191,14 @@ impl Exchange {
     {
         match list {
             ComposeRecipientList::Single(recipient) => {
-                writeln!(w, "{}: {}", name, recipient.to_header_value()?)?;
+                writeln_crlf!(w, "{}: {}", name, recipient.to_header_value()?)?;
             }
             ComposeRecipientList::Multiple(recipients) if recipients.is_empty() => {
-                writeln!(w, "{}: ", name)?;
+                writeln_crlf!(w, "{}: ", name)?;
             }
             ComposeRecipientList::Multiple(recipients) => {
                 for recipient in recipients {
-                    writeln!(w, "{}: {}", name, recipient.to_header_value()?)?;
+                    writeln_crlf!(w, "{}: {}", name, recipient.to_header_value()?)?;
                 }
             }
         }
@@ -208,7 +210,7 @@ impl Exchange {
         W: io::Write,
     {
         for line in HEADER_HELP_LINES {
-            writeln!(w, "{}: {}", HEADER_HELP, line)?;
+            writeln_crlf!(w, "{}: {}", HEADER_HELP, line)?;
         }
         Ok(())
     }
@@ -254,6 +256,7 @@ mod tests {
         assert!(output.contains("X-ExtEditorR-Send-On-Exit: false"));
         assert!(output.ends_with(&request.compose_details.plain_text_body));
         assert!(!output.contains(&request.compose_details.body));
+        assert_eq!(output.matches('\r').count(), output.matches('\n').count());
     }
 
     #[test]

src/model/thunderbird.rs

@@ -76,7 +76,18 @@ pub struct ComposeDetails {
 }
 
 impl ComposeDetails {
+    #[cfg(not(target_os = "windows"))]
+    pub fn get_body(&self) -> String {
+        if self.is_plain_text {
+            self.plain_text_body.replace('\n', "\r\n")
+        } else {
+            self.body.replace('\n', "\r\n")
+        }
+    }
+
+    #[cfg(target_os = "windows")]
     pub fn get_body(&self) -> &str {
+        // Thunderbird under Windows already sends CRLF
         if self.is_plain_text {
             &self.plain_text_body
         } else {
@@ -327,6 +338,21 @@ mod tests {
         }
     }
 
+    #[test]
+    fn compose_details_crlf_body_test() {
+        let mut compose_details = get_blank_compose_details();
+        compose_details.plain_text_body = if cfg!(target_os = "windows") {
+            "Hello,\r\nworld!".to_owned()
+        } else {
+            "Hello,\nworld!".to_owned()
+        };
+
+        let body = compose_details.get_body();
+        assert_eq!(1, body.matches("\r\n").count());
+        assert_eq!(1, body.matches('\r').count());
+        assert_eq!(1, body.matches('\n').count());
+    }
+
     #[test]
     fn compose_details_add_recipient_to_single_test() {
         let mut compose_details = get_blank_compose_details();

src/util.rs

@@ -4,6 +4,16 @@ use std::path::{Path, PathBuf};
 
 use crate::model::thunderbird::Tab;
 
+#[macro_export]
+macro_rules! writeln_crlf {
+    ($dst:expr $(,)?) => {
+        write!($dst, "\r\n")
+    };
+    ($dst:expr, $fmt:expr, $($arg:tt)*) => {
+        write!($dst, concat!($fmt, "\r\n"), $($arg)*)
+    };
+}
+
 pub fn get_temp_filename(tab: &Tab) -> PathBuf {
     let mut temp_dir = env::temp_dir();
     temp_dir.push(format!("external_editor_revived_{}.eml", tab.id));