Fix lzma write support to handle "try again" case

The liblzma-based write code needs the same tweaks that the read code
already has to cope with the situation where lzma_code returns zero the
first time through because avail_out is zero, but will do more work if
called again.

This ports the read tweaks to the write code as closely as possible
(including matching comments etc.).

Closes: #751688

apt-pkg/contrib/fileutl.cc

@@ -1816,6 +1816,7 @@ public:
    }
    virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
    {
+      ssize_t Res;
       lzma->stream.next_in = (uint8_t *)From;
       lzma->stream.avail_in = Size;
       lzma->stream.next_out = lzma->buffer;
@@ -1826,9 +1827,21 @@ public:
       size_t const n = sizeof(lzma->buffer)/sizeof(lzma->buffer[0]) - lzma->stream.avail_out;
       size_t const m = (n == 0) ? 0 : fwrite(lzma->buffer, 1, n, lzma->file);
       if (m != n)
-	 return -1;
+      {
+	 Res = -1;
+	 errno = 0;
+      }
       else
-	 return Size - lzma->stream.avail_in;
+      {
+	 Res = Size - lzma->stream.avail_in;
+	 if (Res == 0)
+	 {
+	    // lzma run was okay, but produced no output…
+	    Res = -1;
+	    errno = EINTR;
+	 }
+      }
+      return Res;
    }
    virtual bool InternalWriteError() override
    {
@@ -2425,10 +2438,18 @@ bool FileFd::Write(const void *From,unsigned long long Size)
    while (Res > 0 && Size > 0)
    {
       Res = d->InternalWrite(From, Size);
-      if (Res < 0 && errno == EINTR)
-	 continue;
+
       if (Res < 0)
+      {
+	 if (errno == EINTR)
+	 {
+	    // trick the while-loop into running again
+	    Res = 1;
+	    errno = 0;
+	    continue;
+	 }
 	 return d->InternalWriteError();
+      }
 
       From = (char const *)From + Res;
       Size -= Res;