Make deepMatch report in linear time

The current implementation of the deepMatch() has the exponential runtime.
Given the long enough input and the pattern with multiple wildcards
it takes a while if ever to complete which can potentially be used
maliciously to cause a denial of service (cpu and memory consumption).

E.g. running this in the root of this repository
time yq eval '.jobs.publishDocker.steps.[] | select (.run == "****outputs")' .github/workflows/release.yml
gives on my laptop
25.11s user 0.06s system 99% cpu 25.182 total

Whereas the updated implementation gives
0.01s user 0.01s system 36% cpu 0.049 total

There are numerous similar CVEs reported for glob evaluation in
different shells/ftp-servers/libraries.

The replacement implementation with the linear runtime is shamelessly taken
verbatim from the briliant article by Russ Cox https://research.swtch.com/glob

pkg/yqlib/matchKeyString.go

@@ -9,26 +9,50 @@ func matchKey(name string, pattern string) (matched bool) {
 		log.Debug("wild!")
 		return true
 	}
-	return deepMatch([]rune(name), []rune(pattern))
+	return deepMatch(name, pattern)
 }
 
-func deepMatch(str, pattern []rune) bool {
+// deepMatch reports whether the name matches the pattern in linear time.
-	for len(pattern) > 0 {
+// Source https://research.swtch.com/glob
-		switch pattern[0] {
+func deepMatch(name, pattern string) bool {
-		default:
+	px := 0
-			if len(str) == 0 || str[0] != pattern[0] {
+	nx := 0
+	nextPx := 0
+	nextNx := 0
+	for px < len(pattern) || nx < len(name) {
+		if px < len(pattern) {
+			c := pattern[px]
+			switch c {
+			default: // ordinary character
+				if nx < len(name) && name[nx] == c {
+					px++
+					nx++
+					continue
+				}
+			case '?': // single-character wildcard
+				if nx < len(name) {
+					px++
+					nx++
+					continue
+				}
+			case '*': // zero-or-more-character wildcard
+				// Try to match at nx.
+				// If that doesn't work out,
+				// restart at nx+1 next.
+				nextPx = px
+				nextNx = nx + 1
+				px++
+				continue
+			}
+		}
+		// Mismatch. Maybe restart.
+		if 0 < nextNx && nextNx <= len(name) {
+			px = nextPx
+			nx = nextNx
+			continue
+		}
 		return false
 	}
-		case '?':
+	// Matched all of pattern to all of name. Success.
-			if len(str) == 0 {
+	return true
-				return false
-			}
-		case '*':
-			return deepMatch(str, pattern[1:]) ||
-				(len(str) > 0 && deepMatch(str[1:], pattern))
-		}
-		str = str[1:]
-		pattern = pattern[1:]
-	}
-	return len(str) == 0 && len(pattern) == 0
 }

pkg/yqlib/matchKeyString_test.go

@@ -0,0 +1,40 @@
+package yqlib
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDeepMatch(t *testing.T) {
+	var tests = []struct {
+		name    string
+		pattern string
+		ok      bool
+	}{
+		{"", "", true},
+		{"", "x", false},
+		{"x", "", false},
+		{"abc", "abc", true},
+		{"abc", "*", true},
+		{"abc", "*c", true},
+		{"abc", "*b", false},
+		{"abc", "a*", true},
+		{"abc", "b*", false},
+		{"a", "a*", true},
+		{"a", "*a", true},
+		{"axbxcxdxe", "a*b*c*d*e*", true},
+		{"axbxcxdxexxx", "a*b*c*d*e*", true},
+		{"abxbbxdbxebxczzx", "a*b?c*x", true},
+		{"abxbbxdbxebxczzy", "a*b?c*x", false},
+		{strings.Repeat("a", 100), "a*a*a*a*b", false},
+		{"xxx", "*x", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name+" "+tt.pattern, func(t *testing.T) {
+			if want, got := tt.ok, deepMatch(tt.name, tt.pattern); want != got {
+				t.Errorf("Expected %v got %v", want, got)
+			}
+		})
+	}
+}