From 2f62c430e0061a2ead7bc81fe085d0e790b05c88 Mon Sep 17 00:00:00 2001
From: Kevin Burke <kevin@meter.com>
Date: Tue, 28 Jan 2020 14:25:27 -0800
Subject: [PATCH] url: reduce allocations in ParseURL

If you use e.g. "postgres://" as the connection string, ParseURL will
be called and the result will be computed each time a connection is
established to the database. This can be expensive because a new
strings.Replacer is created each time ParseURL is called. It's also
unnecessary because the output can be computed by the input and for
most processes only a single input will ever exist for the lifetime of
the process.

Reuse the same *Replacer across calls, and also cache the first 100
unique results from ParseURL, to speed up the second and third calls.
---
 url.go | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/url.go b/url.go
index f4d8a7c2..609c37a8 100644
--- a/url.go
+++ b/url.go
@@ -6,8 +6,15 @@ import (
 	nurl "net/url"
 	"sort"
 	"strings"
+	"sync"
 )
 
+var escaperOnce sync.Once
+var escaper *strings.Replacer
+
+var parsedURLCache map[string]string
+var parsedURLMu sync.Mutex
+
 // ParseURL no longer needs to be used by clients of this library since supplying a URL as a
 // connection string to sql.Open() is now supported:
 //
@@ -30,6 +37,12 @@ import (
 //
 // This will be blank, causing driver.Open to use all of the defaults
 func ParseURL(url string) (string, error) {
+	parsedURLMu.Lock()
+	cachedVal, ok := parsedURLCache[url]
+	parsedURLMu.Unlock()
+	if ok {
+		return cachedVal, nil
+	}
 	u, err := nurl.Parse(url)
 	if err != nil {
 		return "", err
@@ -40,7 +53,9 @@ func ParseURL(url string) (string, error) {
 	}
 
 	var kvs []string
-	escaper := strings.NewReplacer(` `, `\ `, `'`, `\'`, `\`, `\\`)
+	escaperOnce.Do(func() {
+		escaper = strings.NewReplacer(` `, `\ `, `'`, `\'`, `\`, `\\`)
+	})
 	accrue := func(k, v string) {
 		if v != "" {
 			kvs = append(kvs, k+"="+escaper.Replace(v))
@@ -72,5 +87,15 @@ func ParseURL(url string) (string, error) {
 	}
 
 	sort.Strings(kvs) // Makes testing easier (not a performance concern)
-	return strings.Join(kvs, " "), nil
+	result := strings.Join(kvs, " ")
+	parsedURLMu.Lock()
+	if parsedURLCache == nil {
+		parsedURLCache = make(map[string]string)
+	}
+	// don't take up an unbounded amount of memory
+	if len(parsedURLCache) < 100 {
+		parsedURLCache[url] = result
+	}
+	parsedURLMu.Unlock()
+	return result, nil
 }