From 54150b54cd06e3deba44e73b151070b74a4d8e76 Mon Sep 17 00:00:00 2001
From: Nick White <git@njw.name>
Date: Tue, 26 Jan 2021 14:17:19 +0000
Subject: Improve lspipeline concurrency by removing WaitGroup stuff

---
 cmd/lspipeline/main.go | 52 ++++++++++++++++++++------------------------------
 1 file changed, 21 insertions(+), 31 deletions(-)

diff --git a/cmd/lspipeline/main.go b/cmd/lspipeline/main.go
index bf19b52..8980c59 100644
--- a/cmd/lspipeline/main.go
+++ b/cmd/lspipeline/main.go
@@ -12,7 +12,6 @@ import (
 	"os/exec"
 	"sort"
 	"strings"
-	"sync"
 	"time"
 
 	"rescribe.xyz/bookpipeline"
@@ -124,11 +123,10 @@ func getBookDetails(conn LsPipeliner, key string) (date time.Time, done bool, er
 	return objs[0].Date, false, nil
 }
 
-// getBookDetailsWg gets the details for a book putting it into either the
-// done or inprogress channels as appropriate, and using a sync.WaitGroup Done
-// signal so it can be tracked. On error it sends to the errc channel.
-func getBookDetailsWg(conn LsPipeliner, key string, done chan bookpipeline.ObjMeta, inprogress chan bookpipeline.ObjMeta, errc chan error, wg *sync.WaitGroup) {
-	defer wg.Done()
+// getBookDetailsChan gets the details for a book putting it into either the
+// done or inprogress channels as appropriate, or sending an error to errc
+// on failure.
+func getBookDetailsChan(conn LsPipeliner, key string, done chan bookpipeline.ObjMeta, inprogress chan bookpipeline.ObjMeta, errc chan error) {
 	date, isdone, err := getBookDetails(conn, key)
 	if err != nil {
 		errc <- err
@@ -148,6 +146,8 @@ func getBookDetailsWg(conn LsPipeliner, key string, done chan bookpipeline.ObjMe
 // list), and those which do not (the inprogress list). They are
 // sorted according to the date of the graph.png file, or the date
 // of a random file with the prefix if no graph.png was found.
+// It spins up many goroutines to do query the book status and
+// dates, as it is far faster to do concurrently.
 func getBookStatus(conn LsPipeliner) (inprogress []string, done []string, err error) {
 	prefixes, err := conn.ListObjectPrefixes(conn.WIPStorageId())
 	if err != nil {
@@ -155,37 +155,27 @@ func getBookStatus(conn LsPipeliner) (inprogress []string, done []string, err er
 		return
 	}
 
-	// 100,000 size buffer is to ensure we never block, as we're using waitgroup
-	// rather than channel blocking to determine when to continue. Probably there
-	// is a better way to do this, though, like reading the done and inprogress
-	// channels in a goroutine and doing wg.Done() when each is read there instead.
-	donec := make(chan bookpipeline.ObjMeta, 100000)
-	inprogressc := make(chan bookpipeline.ObjMeta, 100000)
-	errc := make(chan error, 100000)
+	donec := make(chan bookpipeline.ObjMeta, 100)
+	inprogressc := make(chan bookpipeline.ObjMeta, 100)
+	errc := make(chan error)
 
-	var wg sync.WaitGroup
 	for _, p := range prefixes {
-		wg.Add(1)
-		go getBookDetailsWg(conn, p, donec, inprogressc, errc, &wg)
-	}
-	wg.Wait()
-	close(donec)
-	close(inprogressc)
-
-	select {
-		case err = <-errc:
-			return inprogress, done, err
-		default:
-			break
+		go getBookDetailsChan(conn, p, donec, inprogressc, errc)
 	}
 
 	var inprogressmeta, donemeta ObjMetas
 
-	for i := range donec {
-		donemeta = append(donemeta, i)
-	}
-	for i := range inprogressc {
-		inprogressmeta = append(inprogressmeta, i)
+	// there will be exactly as many sends to donec or inprogressc
+	// as there are prefixes
+	for range prefixes {
+		select {
+		case i := <-donec:
+			donemeta = append(donemeta, i)
+		case i := <-inprogressc:
+			inprogressmeta = append(inprogressmeta, i)
+		case err = <-errc:
+			return inprogress, done, err
+		}
 	}
 
 	sort.Sort(donemeta)
-- 
cgit v1.2.1-24-ge1ad