Escape PostgreSQL options (#3800)

* Escape PostgreSQL options

* Use raw string literals in test case

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

* Document escaping behavior for options

* Remove heap allocations for options formatting

* Use an actual config option for the test case

---------

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

sqlx-postgres/src/options/mod.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 use std::env::var;
-use std::fmt::{Display, Write};
+use std::fmt::{self, Display, Write};
 use std::path::{Path, PathBuf};
 
 pub use ssl_mode::PgSslMode;
@@ -416,6 +416,9 @@ impl PgConnectOptions {
 
     /// Set additional startup options for the connection as a list of key-value pairs.
     ///
+    /// Escapes the options’ backslash and space characters as per
+    /// https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-OPTIONS
+    ///
     /// # Example
     ///
     /// ```rust
@@ -436,7 +439,8 @@ impl PgConnectOptions {
                 options_str.push(' ');
             }
 
-            write!(options_str, "-c {k}={v}").expect("failed to write an option to the string");
+            options_str.push_str("-c ");
+            write!(PgOptionsWriteEscaped(options_str), "{k}={v}").ok();
         }
         self
     }
@@ -590,6 +594,39 @@ fn default_host(port: u16) -> String {
     "localhost".to_owned()
 }
 
+/// Writer that escapes passed-in PostgreSQL options.
+///
+/// Escapes backslashes and spaces with an additional backslash according to
+/// https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-OPTIONS
+#[derive(Debug)]
+struct PgOptionsWriteEscaped<'a>(&'a mut String);
+
+impl Write for PgOptionsWriteEscaped<'_> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let mut span_start = 0;
+
+        for (span_end, matched) in s.match_indices([' ', '\\']) {
+            write!(self.0, r"{}\{matched}", &s[span_start..span_end])?;
+            span_start = span_end + matched.len();
+        }
+
+        // Write the rest of the string after the last match, or all of it if no matches
+        self.0.push_str(&s[span_start..]);
+
+        Ok(())
+    }
+
+    fn write_char(&mut self, ch: char) -> fmt::Result {
+        if matches!(ch, ' ' | '\\') {
+            self.0.push('\\');
+        }
+
+        self.0.push(ch);
+
+        Ok(())
+    }
+}
+
 #[test]
 fn test_options_formatting() {
     let options = PgConnectOptions::new().options([("geqo", "off")]);
@@ -604,6 +641,26 @@ fn test_options_formatting() {
         options.options,
         Some("-c geqo=off -c statement_timeout=5min".to_string())
     );
+    // https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-OPTIONS
+    let options =
+        PgConnectOptions::new().options([("application_name", r"/back\slash/ and\ spaces")]);
+    assert_eq!(
+        options.options,
+        Some(r"-c application_name=/back\\slash/\ and\\\ spaces".to_string())
+    );
     let options = PgConnectOptions::new();
     assert_eq!(options.options, None);
 }
+
+#[test]
+fn test_pg_write_escaped() {
+    let mut buf = String::new();
+    let mut x = PgOptionsWriteEscaped(&mut buf);
+    x.write_str("x").unwrap();
+    x.write_str("").unwrap();
+    x.write_char('\\').unwrap();
+    x.write_str("y \\").unwrap();
+    x.write_char(' ').unwrap();
+    x.write_char('z').unwrap();
+    assert_eq!(buf, r"x\\y\ \\\ z");
+}