go/store/nbs: aws_table_persister.go: Fix conjoin code to not error on a single short Read() by using io.ReadFull.

In #5307 we started returning a real io.Reader for chunkSource.reader(ctx).
That means the Read() calls can come back short if they are directly attached
to a net.TCPConn, for example. It is not an error for a Read() to come back
short. Use io.ReadFull to correctly express the intent in the conjoin code for
AWS remotes.

Fixes a bug where doltremoteapi fails commits sometimes.

go/store/nbs/aws_table_persister.go

@@ -25,6 +25,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"net/url"
 	"sort"
@@ -392,9 +393,9 @@ func (s3p awsTablePersister) assembleTable(ctx context.Context, plan compactionP
 		readWg.Add(1)
 		go func(m manualPart) {
 			defer readWg.Done()
-			n, _ := m.srcR.Read(buff[m.dstStart:m.dstEnd])
-			if int64(n) < m.dstEnd-m.dstStart {
-				ae.SetIfError(errors.New("failed to read all the table data"))
+			err := m.run(ctx, buff)
+			if err != nil {
+				ae.SetIfError(fmt.Errorf("failed to read conjoin table data: %w", err))
 			}
 		}(man)
 	}
@@ -507,8 +508,17 @@ type copyPart struct {
 }
 
 type manualPart struct {
-	srcR             io.Reader
-	dstStart, dstEnd int64
+	src        chunkSource
+	start, end int64
+}
+
+func (mp manualPart) run(ctx context.Context, buff []byte) error {
+	reader, _, err := mp.src.reader(ctx)
+	if err != nil {
+		return err
+	}
+	_, err = io.ReadFull(reader, buff[mp.start:mp.end])
+	return err
 }
 
 // dividePlan assumes that plan.sources (which is of type chunkSourcesByDescendingDataSize) is correctly sorted by descending data size.
@@ -545,12 +555,7 @@ func dividePlan(ctx context.Context, plan compactionPlan, minPartSize, maxPartSi
 	var offset int64
 	for ; i < len(plan.sources.sws); i++ {
 		sws := plan.sources.sws[i]
-		rdr, _, err := sws.source.reader(ctx)
-		if err != nil {
-			return nil, nil, nil, err
-		}
-
-		manuals = append(manuals, manualPart{rdr, offset, offset + int64(sws.dataLen)})
+		manuals = append(manuals, manualPart{sws.source, offset, offset + int64(sws.dataLen)})
 		offset += int64(sws.dataLen)
 		buffSize += sws.dataLen
 	}

go/store/nbs/aws_table_persister_test.go

@@ -304,7 +304,7 @@ func TestAWSTablePersisterDividePlan(t *testing.T) {
 	assert.Len(manuals, 1)
 	ti, err = tooSmall.index()
 	require.NoError(t, err)
-	assert.EqualValues(calcChunkRangeSize(ti), manuals[0].dstEnd-manuals[0].dstStart)
+	assert.EqualValues(calcChunkRangeSize(ti), manuals[0].end-manuals[0].start)
 }
 
 func TestAWSTablePersisterCalcPartSizes(t *testing.T) {