Addressing issue brought up by dsymonds:

- When providing alternative spellings to a query, do not
  prefix it with a package qualifier as the suggestion may
  not have any results. Correctly filtering is quite a bit
  of work, and clicking the alternative spelling will always
  also show the qualified hits if they exist (but also others).
  Seems good enough for now.
- Give user feedback when the query syntax was wrong.
- Package names in search results are now links to the respective
  package documentation.
- Experimented with excluding main packages and test files
  from index with inconclusive results. Code is present and
  can be enabled by changing a flag in the source. This needs
  some more work.

R=rsc
CC=r, dsymonds
http://go/go-review/1026033

lib/godoc/search.html

@@ -22,7 +22,7 @@
 	{.section Decls}
 		<h2>Package-level declarations</h2>
 		{.repeated section @}
-			<h3>package {Pak.Name|html}</h3>
+			<h3>package <a href="{Pak.Path|path}" class="noline">{Pak.Name|html}</a></h3>
 			{.repeated section Files}
 				{.repeated section Groups}
 					{.repeated section Infos}
@@ -36,7 +36,7 @@
 	{.section Others}
 		<h2>Local declarations and uses</h2>
 		{.repeated section @}
-			<h3>package {Pak.Name|html}</h3>
+			<h3>package <a href="{Pak.Path|path}" class="noline">{Pak.Name|html}</a></h3>
 			{.repeated section Files}
 				<a href="{File.Path|html}?h={Query|html}" class="noline">{File.Path|html}</a>
 				<table class="layout">
@@ -56,7 +56,11 @@
 			{.end}
 		{.end}
 	{.end}
-{.or}
+{.end}
+{.section Illegal}
+	<p>
+	<span class="alert" style="font-size:120%">Illegal query syntax</span>
+	</p>
 	<p>
 	A legal query is a single identifier (such as <a href="search?q=ToLower">ToLower</a>)
 	or a qualified identifier (such as <a href="search?q=math.Sin">math.Sin</a>).

src/cmd/godoc/godoc.go

@@ -1088,6 +1088,7 @@ type SearchResult struct {
 	Query		string;
 	Hit		*LookupResult;
 	Alt		*AltWords;
+	Illegal		bool;
 	Accurate	bool;
 }
 
@@ -1097,7 +1098,7 @@ func search(c *http.Conn, r *http.Request) {
 
 	if index, timestamp := searchIndex.get(); index != nil {
 		result.Query = query;
-		result.Hit, result.Alt = index.(*Index).Lookup(query);
+		result.Hit, result.Alt, result.Illegal = index.(*Index).Lookup(query);
 		_, ts := fsTree.get();
 		result.Accurate = timestamp >= ts;
 	}

src/cmd/godoc/index.go

@@ -29,6 +29,7 @@ import (
 	"go/ast";
 	"go/parser";
 	"go/token";
+	"go/scanner";
 	"os";
 	pathutil "path";
 	"sort";
@@ -411,6 +412,11 @@ func (a *AltWords) filter(s string) *AltWords {
 // ----------------------------------------------------------------------------
 // Indexer
 
+// Adjust these flags as seems best.
+const excludeMainPackages = false
+const excludeTestFiles = false
+
+
 type IndexResult struct {
 	Decls	RunList;	// package-level declarations (with snippets)
 	Others	RunList;	// all other occurences
@@ -583,6 +589,14 @@ func (x *Indexer) VisitFile(path string, d *os.Dir) {
 		return;
 	}
 
+	if excludeTestFiles && (!isPkgFile(d) || strings.HasPrefix(path, "test/")) {
+		return;
+	}
+
+	if excludeMainPackages && pkgName(path) == "main" {
+		return;
+	}
+
 	file, err := parser.ParseFile(path, nil, parser.ParseComments);
 	if err != nil {
 		return;	// ignore files with (parse) errors
@@ -681,11 +695,30 @@ func (x *Index) LookupWord(w string) (match *LookupResult, alt *AltWords) {
 }
 
 
-// For a given string s, which is either a single identifier or a qualified
+func isIdentifier(s string) bool {
+	var S scanner.Scanner;
+	S.Init("", strings.Bytes(s), nil, 0);
+	if _, tok, _ := S.Scan(); tok == token.IDENT {
+		_, tok, _ := S.Scan();
+		return tok == token.EOF;
+	}
+	return false;
+}
+
+
+// For a given query, which is either a single identifier or a qualified
 // identifier, Lookup returns a LookupResult, and a list of alternative
-// spellings, if any.
-func (x *Index) Lookup(s string) (match *LookupResult, alt *AltWords) {
-	ss := strings.Split(s, ".", 0);
+// spellings, if any. If the query syntax is wrong, illegal is set.
+func (x *Index) Lookup(query string) (match *LookupResult, alt *AltWords, illegal bool) {
+	ss := strings.Split(query, ".", 0);
+
+	// check query syntax
+	for _, s := range ss {
+		if !isIdentifier(s) {
+			illegal = true;
+			return;
+		}
+	}
 
 	switch len(ss) {
 	case 1:
@@ -700,18 +733,9 @@ func (x *Index) Lookup(s string) (match *LookupResult, alt *AltWords) {
 			others := match.Others.filter(pakname);
 			match = &LookupResult{decls, others};
 		}
-		if alt != nil {
-			// alternative spellings found - add package name
-			// TODO(gri): At the moment this is not very smart
-			// and likely will produce suggestions that have
-			// no match. Should filter incorrect alternatives.
-			canon := pakname + "." + alt.Canon;	// for completeness (currently not used)
-			alts := make([]string, len(alt.Alts));
-			for i, a := range alt.Alts {
-				alts[i] = pakname+"."+a;
-			}
-			alt = &AltWords{canon, alts};
-		}
+
+	default:
+		illegal = true;
 	}
 
 	return;