[PATCH v3 5/8] virfdstream: Allow sparse stream vol-download

[Michal Privoznik]

When handling sparse stream, a thread is executed. This thread
runs a read() or write() loop (depending what API is called; in
this case it's virStorageVolDownload() and  this the thread run
read() loop). The read() is handled in virFDStreamThreadDoRead()
which is then data/hole section aware, meaning it uses
virFileInData() to detect data and hole sections and sends
TYPE_DATA or TYPE_HOLE virStream messages accordingly.

However, virFileInData() does not work with block devices. Simply
because block devices don't have data and hole sections. What we
can do though, is to mimic being always in a DATA section.

Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1852528

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
Reviewed-by: Peter Krempa <pkrempa at redhat.com>
---
 src/util/virfdstream.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/util/virfdstream.c b/src/util/virfdstream.c
index 44399b2dc0..39514ef555 100644
--- a/src/util/virfdstream.c
+++ b/src/util/virfdstream.c
@@ -398,6 +398,7 @@ struct _virFDStreamThreadData {
     size_t length;
     bool doRead;
     bool sparse;
+    bool isBlock;
     int fdin;
     char *fdinname;
     int fdout;
@@ -421,6 +422,7 @@ virFDStreamThreadDataFree(virFDStreamThreadDataPtr data)
 static ssize_t
 virFDStreamThreadDoRead(virFDStreamDataPtr fdst,
                         bool sparse,
+                        bool isBlock,
                         const int fdin,
                         const int fdout,
                         const char *fdinname,
@@ -437,8 +439,20 @@ virFDStreamThreadDoRead(virFDStreamDataPtr fdst,
     ssize_t got;
 
     if (sparse && *dataLen == 0) {
-        if (virFileInData(fdin, &inData, &sectionLen) < 0)
-            return -1;
+        if (isBlock) {
+            /* Block devices are always in data section by definition. The
+             * @sectionLen is slightly more tricky. While we could try and get
+             * how much bytes is there left until EOF, we can pretend there is
+             * always X bytes left and let the saferead() below hit EOF (which
+             * is then handled gracefully anyway). Worst case scenario, this
+             * branch is called more than once.
+             * X was chosen to be 1MiB but it has ho special meaning. */
+            inData = 1;
+            sectionLen = 1 * 1024 * 1024;
+        } else {
+            if (virFileInData(fdin, &inData, &sectionLen) < 0)
+                return -1;
+        }
 
         if (length &&
             sectionLen > length - total)
@@ -568,6 +582,7 @@ virFDStreamThread(void *opaque)
     virStreamPtr st = data->st;
     size_t length = data->length;
     bool sparse = data->sparse;
+    bool isBlock = data->isBlock;
     VIR_AUTOCLOSE fdin = data->fdin;
     char *fdinname = data->fdinname;
     VIR_AUTOCLOSE fdout = data->fdout;
@@ -604,7 +619,7 @@ virFDStreamThread(void *opaque)
         }
 
         if (doRead)
-            got = virFDStreamThreadDoRead(fdst, sparse,
+            got = virFDStreamThreadDoRead(fdst, sparse, isBlock,
                                           fdin, fdout,
                                           fdinname, fdoutname,
                                           length, total,
@@ -1271,6 +1286,7 @@ virFDStreamOpenFileInternal(virStreamPtr st,
         threadData->st = virObjectRef(st);
         threadData->length = length;
         threadData->sparse = sparse;
+        threadData->isBlock = !!S_ISBLK(sb.st_mode);
 
         if ((oflags & O_ACCMODE) == O_RDONLY) {
             threadData->fdin = fd;
-- 
2.26.2