[PATCH 3/9] virfile: Introduce virFileInDataDetectZeroes()
For given file descriptor determine if the current position it is
in plus 1MiB (arbitrary chosen value) consists solely from zero
bytes or not. This is a block device friendly version of
virFileInData().
Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com>
---
src/libvirt_private.syms | 1 +
src/util/virfile.c | 67 ++++++++++++++++++++++++++++++++++++++++
src/util/virfile.h | 4 +++
3 files changed, 72 insertions(+)
diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 1d80aeb833..6b5a751788 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -2070,6 +2070,7 @@ virFileGetMountSubtree;
virFileGetXAttr;
virFileGetXAttrQuiet;
virFileInData;
+virFileInDataDetectZeroes;
virFileIsCDROM;
virFileIsDir;
virFileIsExecutable;
diff --git a/src/util/virfile.c b/src/util/virfile.c
index c034df5931..18b5096b88 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -4064,6 +4064,73 @@ virFileInData(int fd G_GNUC_UNUSED,
#endif /* !HAVE_DECL_SEEK_HOLE */
+#define DETECT_ZEORES_BLOCK_SIZE (1 * 1024 * 1024)
+
+/*
+ * virFileInDataDetectZeroes:
+ * @fd: file to check
+ * @inData: true if current position in the @fd is in data section
+ * @length: amount of bytes until the end of the current section
+ *
+ * This behaves exactly like virFileInData() except it doesn't use SEEK_DATA
+ * and SEEK_HOLE rather than a buffer into which it reads data from @fd and
+ * detects if the buffer is full of zeroes. Therefore, it is safe to use on
+ * special files (e.g. block devices). On the other hand it sees only
+ * DETECT_ZEORES_BLOCK_SIZE ahead.
+ *
+ * Returns: 0 on success,
+ * -1 otherwise.
+ */
+int
+virFileInDataDetectZeroes(int fd,
+ int *inData,
+ long long *length)
+{
+ const size_t buflen = DETECT_ZEORES_BLOCK_SIZE;
+ g_autofree char *buf = NULL;
+ off_t cur;
+ ssize_t r;
+ int ret = -1;
+
+ /* Get current position */
+ cur = lseek(fd, 0, SEEK_CUR);
+ if (cur == (off_t) -1) {
+ virReportSystemError(errno, "%s",
+ _("Unable to get current position in file"));
+ goto cleanup;
+ }
+
+ buf = g_new0(char, buflen);
+
+ if ((r = saferead(fd, buf, buflen)) < 0) {
+ virReportSystemError(errno, "%s",
+ _("Unable to read from file"));
+ goto cleanup;
+ }
+
+ *inData = !virStringIsNull(buf, r);
+ *length = MIN(r, buflen);
+ ret = 0;
+
+ cleanup:
+ /* At any rate, reposition back to where we started. */
+ if (cur != (off_t) -1) {
+ int theerrno = errno;
+
+ if (lseek(fd, cur, SEEK_SET) == (off_t) -1) {
+ virReportSystemError(errno, "%s",
+ _("unable to restore position in file"));
+ ret = -1;
+ if (theerrno == 0)
+ theerrno = errno;
+ }
+
+ errno = theerrno;
+ }
+ return ret;
+}
+
+
/**
* virFileReadValueInt:
* @value: pointer to int to be filled in with the value
diff --git a/src/util/virfile.h b/src/util/virfile.h
index 7a92364a5c..9a5eade609 100644
--- a/src/util/virfile.h
+++ b/src/util/virfile.h
@@ -352,6 +352,10 @@ int virFileInData(int fd,
int *inData,
long long *length);
+int virFileInDataDetectZeroes(int fd,
+ int *inData,
+ long long *length);
+
G_DEFINE_AUTOPTR_CLEANUP_FUNC(virFileWrapperFd, virFileWrapperFdFree);
int virFileGetXAttr(const char *path,
--
2.26.2