mysqlindexer: implement iterator. untested.

Change-Id: I168e2b6c30c4f8a83c73a3530652f4fb82333cda

pkg/mysqlindexer/mysqlindexer.go

@@ -18,8 +18,11 @@ package mysqlindexer
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
+	"log"
 	"os"
+	"strconv"
 
 	"camlistore.org/pkg/blobserver"
 	"camlistore.org/pkg/index"
@@ -90,7 +93,75 @@ func (ms *myIndexStorage) Delete(key string) error {
 }
 
 func (ms *myIndexStorage) Find(key string) index.Iterator {
-	panic("TODO(bradfitz): implement")
+	return &iter{
+		ms:  ms,
+		low: key,
+		op:  ">=",
+	}
+}
+
+// iter is a iterator over sorted key/value pairs in rows.
+type iter struct {
+	ms  *myIndexStorage
+	low string
+	op  string // ">=" initially, then ">"
+	err error  // accumulated error, returned at Close
+
+	rows *sql.Rows // if non-nil, the rows we're reading from
+
+	batchSize int // how big our LIMIT query was
+	seen      int // how many rows we've seen this query
+
+	key   string
+	value string
+}
+
+var errClosed = errors.New("mysqlindexer: Iterator already closed")
+
+func (t *iter) Key() string   { return t.key }
+func (t *iter) Value() string { return t.value }
+
+func (t *iter) Close() error {
+	if t.rows != nil {
+		t.rows.Close()
+	}
+	err := t.err
+	t.err = errClosed
+	return err
+}
+
+func (t *iter) Next() bool {
+	if t.err != nil {
+		return false
+	}
+	if t.rows == nil {
+		const batchSize = 50
+		t.batchSize = batchSize
+		t.rows, t.err = t.ms.db.Query(
+			"SELECT k, v FROM rows WHERE k "+t.op+" ? ORDER BY k LIMIT "+strconv.Itoa(batchSize),
+			t.low)
+		if t.err != nil {
+			log.Printf("unexpected query error: %v", t.err)
+			return false
+		}
+		t.seen = 0
+		t.op = ">"
+	}
+	if !t.rows.Next() {
+		if t.seen == t.batchSize {
+			t.rows = nil
+			return t.Next()
+		}
+		return false
+	}
+	t.err = t.rows.Scan(&t.key, &t.value)
+	if t.err != nil {
+		log.Printf("unexpected Scan error: %v", t.err)
+		return false
+	}
+	t.low = t.key
+	t.seen++
+	return true
 }
 
 func newFromConfig(ld blobserver.Loader, config jsonconfig.Obj) (blobserver.Storage, error) {