cli-plugins/manager: replace pluginNameRe for isValidPluginName utility

The pluginNameRe was a basic regular expression, effectively only checking
if the name consisted of lowercase alphanumeric characters. Replace it
with a minimal utility to do the same, without having to use regular
expressions (or the "lazyregexp" package).

Some quick benchmarking (not committed) show that the non-regex approach
is ~25x faster:

    BenchmarkIsValidPluginName_Regex_Valid-10       13956240        81.39  ns/op       0 B/op        0 allocs/op
    BenchmarkIsValidPluginName_Manual_Valid-10     360003060         3.318 ns/op       0 B/op        0 allocs/op

    BenchmarkIsValidPluginName_Regex_Invalid-10     35281794        33.74  ns/op       0 B/op        0 allocs/op
    BenchmarkIsValidPluginName_Manual_Invalid-10   906072663         1.320 ns/op       0 B/op        0 allocs/op

    BenchmarkIsValidPluginName_Regex_Parallel-10    96595677        12.04  ns/op       0 B/op        0 allocs/op
    BenchmarkIsValidPluginName_Manual_Parallel-10  1000000000        0.4541 ns/op      0 B/op        0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli-plugins/manager/manager.go

@@ -169,7 +169,7 @@ func PluginRunCommand(dockerCli config.Provider, name string, rootcmd *cobra.Com
 	// have been provided by cobra to our caller. This is because
 	// they lack e.g. global options which we must propagate here.
 	args := os.Args[1:]
-	if !pluginNameRe.MatchString(name) {
+	if !isValidPluginName(name) {
 		// We treat this as "not found" so that callers will
 		// fallback to their "invalid" command path.
 		return nil, errPluginNotFound(name)

cli-plugins/manager/plugin.go

@@ -12,12 +12,9 @@ import (
 	"strings"
 
 	"github.com/docker/cli/cli-plugins/metadata"
-	"github.com/docker/cli/internal/lazyregexp"
 	"github.com/spf13/cobra"
 )
 
-var pluginNameRe = lazyregexp.New("^[a-z][a-z0-9]*$")
-
 // Plugin represents a potential plugin with all it's metadata.
 type Plugin struct {
 	metadata.Metadata
@@ -85,8 +82,8 @@ func newPlugin(c pluginCandidate, cmds []*cobra.Command) (Plugin, error) {
 	}
 
 	// Now apply the candidate tests, so these update p.Err.
-	if !pluginNameRe.MatchString(p.Name) {
-		p.Err = newPluginError("plugin candidate %q did not match %q", p.Name, pluginNameRe.String())
+	if !isValidPluginName(p.Name) {
+		p.Err = newPluginError("plugin candidate %q did not match %q", p.Name, pluginNameFormat)
 		return p, nil
 	}
 
@@ -147,3 +144,26 @@ func (p *Plugin) RunHook(ctx context.Context, hookData HookPluginData) ([]byte,
 
 	return hookCmdOutput, nil
 }
+
+// pluginNameFormat is used as part of errors for invalid plugin-names.
+// We should consider making this less technical ("must start with "a-z",
+// and only consist of lowercase alphanumeric characters").
+const pluginNameFormat = `^[a-z][a-z0-9]*$`
+
+func isValidPluginName(s string) bool {
+	if len(s) == 0 {
+		return false
+	}
+	// first character must be a-z
+	if c := s[0]; c < 'a' || c > 'z' {
+		return false
+	}
+	// followed by a-z or 0-9
+	for i := 1; i < len(s); i++ {
+		c := s[i]
+		if (c < 'a' || c > 'z') && (c < '0' || c > '9') {
+			return false
+		}
+	}
+	return true
+}