fix(writer): escape the escape byte when double_quote is disabled

csv-core/src/writer.rs

@@ -1,7 +1,7 @@
 use core::fmt;
 use core::str;
 
-use memchr::memchr;
+use memchr::{memchr, memchr2};
 
 use crate::{QuoteStyle, Terminator};
 
@@ -547,38 +547,46 @@ pub fn quote(
 ) -> (WriteResult, usize, usize) {
     let (mut nin, mut nout) = (0, 0);
     loop {
-        match memchr(quote, input) {
+        // Find the next byte that must be escaped. When double quoting is
+        // enabled, only the quote byte needs escaping (by doubling it). When
+        // it is disabled, the escape byte itself must also be escaped (by
+        // preceding it with another escape byte); otherwise an escape byte
+        // occurring in the field data would be indistinguishable from the
+        // start of an escape sequence when the data is read back.
+        let next = if double_quote {
+            memchr(quote, input)
+        } else {
+            memchr2(quote, escape, input)
+        };
+        match next {
             None => {
                 let (res, i, o) = write_optimistic(input, output);
                 nin += i;
                 nout += o;
                 return (res, nin, nout);
             }
-            Some(next_quote) => {
-                let (res, i, o) =
-                    write_optimistic(&input[..next_quote], output);
+            Some(next) => {
+                let (res, i, o) = write_optimistic(&input[..next], output);
                 input = &input[i..];
                 output = &mut moving(output)[o..];
                 nin += i;
                 nout += o;
                 if let WriteResult::OutputFull = res {
                     return (res, nin, nout);
                 }
-                if double_quote {
-                    let (res, o) = write_pessimistic(&[quote, quote], output);
-                    if let WriteResult::OutputFull = res {
-                        return (res, nin, nout);
-                    }
-                    nout += o;
-                    output = &mut moving(output)[o..];
+                let escaped = if double_quote {
+                    [quote, quote]
                 } else {
-                    let (res, o) = write_pessimistic(&[escape, quote], output);
-                    if let WriteResult::OutputFull = res {
-                        return (res, nin, nout);
-                    }
-                    nout += o;
-                    output = &mut moving(output)[o..];
+                    // `input[0]` is the byte we found: either the quote byte or
+                    // the escape byte. Both are escaped with the escape byte.
+                    [escape, input[0]]
+                };
+                let (res, o) = write_pessimistic(&escaped, output);
+                if let WriteResult::OutputFull = res {
+                    return (res, nin, nout);
                 }
+                nout += o;
+                output = &mut moving(output)[o..];
                 nin += 1;
                 input = &input[1..];
             }
@@ -1014,6 +1022,35 @@ mod tests {
         assert_quote!(inp, out, 2, 4, InputEmpty, r#"\"\""#, false);
     }
 
+    // When double quoting is disabled, the escape byte itself must be escaped
+    // (by doubling it), otherwise it could not be distinguished from the start
+    // of an escape sequence when the data is read back.
+    #[test]
+    fn quote_escaped_escape_one() {
+        let inp = b(r#"\"#);
+        let out = &mut [0; 1024];
+
+        assert_quote!(inp, out, 1, 2, InputEmpty, r#"\\"#, false);
+    }
+
+    #[test]
+    fn quote_escaped_escape_and_quote() {
+        let inp = b(r#"\""#);
+        let out = &mut [0; 1024];
+
+        assert_quote!(inp, out, 2, 4, InputEmpty, r#"\\\""#, false);
+    }
+
+    // With double quoting enabled (the default), the escape byte is not special
+    // and must be written verbatim.
+    #[test]
+    fn quote_double_quote_leaves_escape_byte() {
+        let inp = b(r#"a\b"#);
+        let out = &mut [0; 1024];
+
+        assert_quote!(inp, out, 3, 3, InputEmpty, r#"a\b"#, true);
+    }
+
     #[test]
     fn quote_misc() {
         let inp = b(r#"foo "bar" baz "quux"?"#);

src/writer.rs

@@ -1294,6 +1294,42 @@ mod tests {
         assert_eq!(wtr_as_string(wtr), "\"\"\n\"\"\n");
     }
 
+    // Regression test: when `double_quote` is disabled, the writer escapes
+    // quotes with the escape character. It must also escape the escape
+    // character itself, otherwise fields containing it (e.g. Windows paths
+    // like `C:\path`) are silently corrupted when read back.
+    #[test]
+    fn escape_escape_char_when_not_double_quoting() {
+        let mut wtr =
+            WriterBuilder::new().double_quote(false).from_writer(vec![]);
+        wtr.write_record([r#"C:\path"#, "END"]).unwrap();
+
+        // The backslash is doubled so the reader can recover it.
+        assert_eq!(wtr_as_string(wtr), "\"C:\\\\path\",END\n");
+    }
+
+    #[test]
+    fn roundtrip_escape_char_when_not_double_quoting() {
+        use crate::ReaderBuilder;
+
+        let mut wtr =
+            WriterBuilder::new().double_quote(false).from_writer(vec![]);
+        let record = [r#"a\b"#, r#"C:\path\to"#, r#"\"#, "plain"];
+        wtr.write_record(record).unwrap();
+        let data = wtr.into_inner().unwrap();
+
+        let mut rdr = ReaderBuilder::new()
+            .has_headers(false)
+            .double_quote(false)
+            .escape(Some(b'\\'))
+            .from_reader(&*data);
+        let got = rdr.records().next().unwrap().unwrap();
+        assert_eq!(
+            got.iter().collect::<Vec<_>>(),
+            vec![r#"a\b"#, r#"C:\path\to"#, r#"\"#, "plain"],
+        );
+    }
+
     #[test]
     fn unequal_records_bad() {
         let mut wtr = WriterBuilder::new().from_writer(vec![]);