Show position in syntax errors

conn.go

@@ -621,7 +621,7 @@ func (cn *conn) simpleExec(q string) (res driver.Result, commandTag string, err
 }
 
 func (cn *conn) simpleQuery(q string) (res *rows, err error) {
-	defer cn.errRecover(&err)
+	defer cn.errRecoverWithQuery(&err, q)
 
 	b := cn.writeBuf('Q')
 	b.string(q)
@@ -763,7 +763,7 @@ func (cn *conn) Prepare(q string) (_ driver.Stmt, err error) {
 	if cn.bad {
 		return nil, driver.ErrBadConn
 	}
-	defer cn.errRecover(&err)
+	defer cn.errRecoverWithQuery(&err, q)
 
 	if len(q) >= 4 && strings.EqualFold(q[:4], "COPY") {
 		return cn.prepareCopyIn(q)
@@ -794,7 +794,7 @@ func (cn *conn) Query(query string, args []driver.Value) (_ driver.Rows, err err
 	if cn.bad {
 		return nil, driver.ErrBadConn
 	}
-	defer cn.errRecover(&err)
+	defer cn.errRecoverWithQuery(&err, query)
 
 	// Check to see if we can use the "simpleQuery" interface, which is
 	// *much* faster than going through prepare/exec
@@ -828,7 +828,7 @@ func (cn *conn) Exec(query string, args []driver.Value) (res driver.Result, err
 	if cn.bad {
 		return nil, driver.ErrBadConn
 	}
-	defer cn.errRecover(&err)
+	defer cn.errRecoverWithQuery(&err, query)
 
 	// Check to see if we can use the "simpleExec" interface, which is
 	// *much* faster than going through prepare/exec

error.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"net"
 	"runtime"
+	"strconv"
+	"strings"
 )
 
 // Error severities
@@ -40,6 +42,9 @@ type Error struct {
 	File             string
 	Line             string
 	Routine          string
+
+	// For syntax error reporting. Not a field returned by Postgres.
+	originalQuery string
 }
 
 // ErrorCode is a five-character error code.
@@ -444,9 +449,61 @@ func (err *Error) Get(k byte) (v string) {
 }
 
 func (err Error) Error() string {
+	switch err.Code {
+	case "42601": // syntax_error
+		return err.syntaxError()
+	default:
+		return err.normalError()
+	}
+}
+
+func (err Error) normalError() string {
 	return "pq: " + err.Message
 }
 
+// syntaxError formats a syntax error the way psql does.
+func (err Error) syntaxError() string {
+	if err.Position == "" || err.originalQuery == "" {
+		return err.normalError() // not enough information, fallback
+	}
+
+	pos, e := strconv.Atoi(err.Position)
+	if e != nil {
+		return err.normalError() // Position is not a number, fallback
+	}
+	pos -= 1 // make zero-based
+
+	if pos < 0 || pos >= len(err.originalQuery) {
+		return err.normalError() // Position is out of range, fallback
+	}
+
+	lineStartPos := strings.LastIndex(err.originalQuery[:pos], "\n")
+	if lineStartPos == -1 { // error in first line?
+		lineStartPos = 0
+	} else {
+		lineStartPos += 1 // remove \n
+	}
+
+	lineEndPos := strings.Index(err.originalQuery[pos:], "\n")
+	if lineEndPos == -1 { // error in last line?
+		lineEndPos = len(err.originalQuery)
+	} else {
+		lineEndPos += pos // absolute position
+	}
+
+	lineNo := strings.Count(err.originalQuery[:lineStartPos], "\n")
+
+	queryLinePrefix := fmt.Sprintf("LINE %d: ", lineNo+1)
+	queryLine := err.originalQuery[lineStartPos:lineEndPos]
+	markerLinePrefix := strings.Repeat(" ", len(queryLinePrefix))
+	markerLine := strings.Repeat(" ", pos-lineStartPos) + "^"
+
+	return fmt.Sprintf("pq: %s\n%s%s\n%s%s",
+		err.Message,
+		queryLinePrefix, queryLine,
+		markerLinePrefix, markerLine)
+}
+
 // PGError is an interface used by previous versions of pq. It is provided
 // only to support legacy code. New code should use the Error type.
 type PGError interface {
@@ -473,8 +530,20 @@ func errRecoverNoErrBadConn(err *error) {
 }
 
 func (c *conn) errRecover(err *error) {
-	e := recover()
-	switch v := e.(type) {
+	c.errHandleRecovered(recover(), err)
+}
+
+func (c *conn) errRecoverWithQuery(err *error, query string) {
+	c.errHandleRecovered(recover(), err)
+	if *err != nil {
+		if pqErr, ok := (*err).(*Error); ok {
+			pqErr.originalQuery = query
+		}
+	}
+}
+
+func (c *conn) errHandleRecovered(recovered interface{}, err *error) {
+	switch v := recovered.(type) {
 	case nil:
 		// Do nothing
 	case runtime.Error:
@@ -497,7 +566,7 @@ func (c *conn) errRecover(err *error) {
 
 	default:
 		c.bad = true
-		panic(fmt.Sprintf("unknown error: %#v", e))
+		panic(fmt.Sprintf("unknown error: %#v", recovered))
 	}
 
 	// Any time we return ErrBadConn, we need to remember it since *Tx doesn't

error_test.go

@@ -0,0 +1,68 @@
+package pq
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSyntaxErrorFormatting(t *testing.T) {
+	for _, tt := range []struct {
+		err      Error
+		expected string
+	}{
+		// Single line
+		{Error{Message: "test", Position: "8", originalQuery: "SELECT *;"},
+			"pq: test\nLINE 1: SELECT *;\n               ^"},
+
+		// Syntax error in first line
+		{Error{Message: "test", Position: "1", originalQuery: "SELECT\n *;"},
+			"pq: test\nLINE 1: SELECT\n        ^"},
+
+		// Syntax error in last line
+		{Error{Message: "test", Position: "9", originalQuery: "SELECT\n *;"},
+			"pq: test\nLINE 2:  *;\n         ^"},
+
+		// Bad input: position non-positive
+		{Error{Message: "test", Position: "0", originalQuery: "SELECT\n *;"},
+			Error{Message: "test", Position: "0", originalQuery: "SELECT\n *;"}.normalError()},
+
+		// Bad input: position after end of string
+		{Error{Message: "test", Position: "11", originalQuery: "SELECT\n *;"},
+			Error{Message: "test", Position: "11", originalQuery: "SELECT\n *;"}.normalError()},
+		{Error{Message: "test", Position: "not a number", originalQuery: "SELECT\n *;"},
+			Error{Message: "test", Position: "not a number", originalQuery: "SELECT\n *;"}.normalError()},
+	} {
+		actual := tt.err.syntaxError()
+		if tt.expected != actual {
+			t.Errorf("bad message, expected %#v, got %#v", tt.expected, actual)
+		}
+	}
+}
+
+func TestSyntaxErrorHandlingWithQuery(t *testing.T) {
+	db := openTestConn(t)
+	defer db.Close()
+
+	_, err := db.Query("SELECT *;")
+	if err == nil {
+		t.Fatal(err)
+	}
+
+	if !strings.HasSuffix(err.Error(), "   ^") {
+		t.Errorf("syntax error not formatted as such. got %#v", err.Error())
+	}
+}
+
+func TestSyntaxErrorHandlingWithPrepare(t *testing.T) {
+	db := openTestConn(t)
+	defer db.Close()
+
+	_, err := db.Prepare("SELECT *;")
+	if err == nil {
+		t.Fatal(err)
+	}
+
+	if !strings.HasSuffix(err.Error(), "   ^") {
+		t.Errorf("syntax error not formatted as such. got %#v", err.Error())
+	}
+}