Added HTTP endpoints for Correctness counts

golden/go/expstorage/sqlexpstore.go

 package expstorage
 
 import (
+        "database/sql"
         "encoding/json"
 
         "skia.googlesource.com/buildbot.git/go/database"
         "skia.googlesource.com/buildbot.git/go/util"
 )
 
 // Stores expectations in an SQL database without any caching.
 type SQLExpectationsStore struct {
         vdb *database.VersionedDB
 }
 
 func NewSQLExpectationStore(vdb *database.VersionedDB) ExpectationsStore {
         return &SQLExpectationsStore{
                 vdb: vdb,
         }
 }
 
 // See ExpectationsStore interface.
 func (e *SQLExpectationsStore) Get(modifiable bool) (exp *Expectations, err error) {
         // Load the newest record from the database.
         stmt := `SELECT expectations
                  FROM expectations
                  ORDER BY ts DESC
                  LIMIT 1`
 
+        // Read the expectations. If there are no rows, that means we have no
+        // expectations yet.
         var expJSON string
-        if err := e.vdb.DB.QueryRow(stmt).Scan(&expJSON); err != nil {
+        switch err := e.vdb.DB.QueryRow(stmt).Scan(&expJSON); {
+        case err == sql.ErrNoRows:
+                return NewExpectations(modifiable), nil
+        case err != nil:
                 return nil, err
         }
 
         var result Expectations
         err = json.Unmarshal([]byte(expJSON), &result)
 
         // Since it's freshly allocated we can just set it to the requested mode.
         result.Modifiable = modifiable
 
         return &result, nil
@@ skipping 48 matching lines @@
 // See ExpectationsStore interface.
 func (c *CachingExpectationStore) Put(exp *Expectations, userId string) error {
         exp.checkModifiable()
 
         if err := c.store.Put(exp, userId); err != nil {
                 return err
         }
 
         return c.cache.Put(exp, userId)
 }