implement reverse reader for log reads

in cases where the log file exceeds the available memory of a system, we had a bug that triggered an oom because the entire logfile was being read when the tail parameter was given.  this reads in chunks and is more or less memory safe.

fixes: #5131

Signed-off-by: Brent Baude <bbaude@redhat.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/libpod/logs/reversereader/reversereader.go b/libpod/logs/reversereader/reversereader.go
new file mode 100644
index 000000000..72d9ad975
--- /dev/null
+++ b/libpod/logs/reversereader/reversereader.go
@@ -0,0 +1,66 @@
+package reversereader
+
+import (
+	"io"
+	"os"
+
+	"github.com/pkg/errors"
+)
+
+// ReverseReader structure for reading a file backwards
+type ReverseReader struct {
+	reader   *os.File
+	offset   int64
+	readSize int64
+}
+
+// NewReverseReader returns a reader that reads from the end of a file
+// rather than the beginning.  It sets the readsize to pagesize and determines
+// the first offset using using modulus.
+func NewReverseReader(reader *os.File) (*ReverseReader, error) {
+	// pagesize should be safe for memory use and file reads should be on page
+	// boundaries as well
+	pageSize := int64(os.Getpagesize())
+	stat, err := reader.Stat()
+	if err != nil {
+		return nil, err
+	}
+	// figure out the last page boundary
+	remainder := stat.Size() % pageSize
+	end, err := reader.Seek(0, 2)
+	if err != nil {
+		return nil, err
+	}
+	// set offset (starting position) to the last page boundary or
+	// zero if fits in one page
+	startOffset := end - remainder
+	if startOffset < 0 {
+		startOffset = 0
+	}
+	rr := ReverseReader{
+		reader:   reader,
+		offset:   startOffset,
+		readSize: pageSize,
+	}
+	return &rr, nil
+}
+
+// ReverseReader reads from a given offset to the previous offset and
+// then sets the newoff set one pagesize less than the previous read.
+func (r *ReverseReader) Read() (string, error) {
+	if r.offset < 0 {
+		return "", errors.Wrap(io.EOF, "at beginning of file")
+	}
+	// Read from given offset
+	b := make([]byte, r.readSize)
+	n, err := r.reader.ReadAt(b, r.offset)
+	if err != nil && errors.Cause(err) != io.EOF {
+		return "", err
+	}
+	if int64(n) < r.readSize {
+		b = b[0:n]
+	}
+	// Set to the next page boundary
+	r.offset = -r.readSize
+	return string(b), nil
+}