cli/context/store: replace restrictedNamePattern for isValidName utility

The restrictedNamePattern was a basic regular expression. 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 ~18x faster:

    BenchmarkIsValidName_Regex_Valid-10        8516511        119.4   ns/op      0 B/op        0 allocs/op
    BenchmarkIsValidName_Manual_Valid-10     172426240          6.964 ns/op      0 B/op        0 allocs/op

    BenchmarkIsValidName_Regex_Invalid-10     34824540         34.22  ns/op      0 B/op        0 allocs/op
    BenchmarkIsValidName_Manual_Invalid-10   550804021          2.173 ns/op      0 B/op        0 allocs/op

    BenchmarkIsValidName_Regex_Parallel-10    69289900         17.30   ns/op     0 B/op        0 allocs/op
    BenchmarkIsValidName_Manual_Parallel-10 1000000000          0.9296 ns/op     0 B/op        0 allocs/op

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

cli/context/store/store.go

@@ -18,14 +18,9 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/docker/cli/internal/lazyregexp"
 	"github.com/opencontainers/go-digest"
 )
 
-const restrictedNamePattern = "^[a-zA-Z0-9][a-zA-Z0-9_.+-]+$"
-
-var restrictedNameRegEx = lazyregexp.New(restrictedNamePattern)
-
 // Store provides a context store for easily remembering endpoints configuration
 type Store interface {
 	Reader
@@ -225,12 +220,43 @@ func ValidateContextName(name string) error {
 	if name == "default" {
 		return errors.New(`"default" is a reserved context name`)
 	}
-	if !restrictedNameRegEx.MatchString(name) {
-		return fmt.Errorf("context name %q is invalid, names are validated against regexp %q", name, restrictedNamePattern)
+	if !isValidName(name) {
+		return fmt.Errorf("context name %q is invalid, names are validated against regexp %q", name, validNameFormat)
 	}
 	return nil
 }
 
+// validNameFormat is used as part of errors for invalid context-names.
+// We should consider making this less technical ("must start with "a-z",
+// and only consist of alphanumeric characters and separators").
+const validNameFormat = `^[a-zA-Z0-9][a-zA-Z0-9_.+-]+$`
+
+// isValidName checks if the context-name is valid ("^[a-zA-Z0-9][a-zA-Z0-9_.+-]+$").
+//
+// Names must start with an alphanumeric character (a-zA-Z0-9), followed by
+// alphanumeric or separators ("_", ".", "+", "-").
+func isValidName(s string) bool {
+	if len(s) < 2 || !isAlphaNum(s[0]) {
+		return false
+	}
+
+	for i := 1; i < len(s); i++ {
+		c := s[i]
+		if isAlphaNum(c) || c == '_' || c == '.' || c == '+' || c == '-' {
+			continue
+		}
+		return false
+	}
+
+	return true
+}
+
+func isAlphaNum(c byte) bool {
+	return (c >= 'a' && c <= 'z') ||
+		(c >= 'A' && c <= 'Z') ||
+		(c >= '0' && c <= '9')
+}
+
 // Export exports an existing namespace into an opaque data stream
 // This stream is actually a tarball containing context metadata and TLS materials, but it does
 // not map 1:1 the layout of the context store (don't try to restore it manually without calling store.Import)