read_header_int: allow leading spaces (fix #111)

Some old versions of `tar` (e.g. Solaris) emitted integer fields with
leadings *spaces* which is technically contrary to the standard, but
seems harmless to support. This adds a fragment of this tarball:

    https://sparse.tamu.edu/MM/Oberwolfach/LF10.tar.gz

This is a collection of example sparse matrix files. The fragment is a
single header entry followed by one block of data and is a valid tarball
by itself with the idiosyncratic field formatting of Solaris `tar`.

src/extract.jl

@@ -601,14 +601,19 @@ end
 
 function read_header_int(buf::AbstractVector{UInt8}, offset::Int, length::Int)
     n = UInt64(0)
-    for i in index_range(offset, length)
+    before = true
+    r = index_range(offset, length)
+    for i in r
         byte = buf[i]
+        before && byte == UInt8(' ') && continue
         byte in (0x00, UInt8(' ')) && break
         UInt8('0') <= byte <= UInt8('7') ||
             error("invalid octal digit: $(repr(Char(byte)))")
         n <<= 3
         n |= byte - 0x30
+        before = false
     end
+    before && error("invalid integer value: $(repr(String(buf[r])))")
     return n
 end
 

test/runtests.jl

@@ -867,3 +867,11 @@ if Sys.iswindows() && Sys.which("icacls") !== nothing && VERSION >= v"1.6"
         end
     end
 end
+
+@testset "leading spaces" begin
+    # fragment of https://sparse.tamu.edu/MM/Oberwolfach/LF10.tar.gz
+    tarball = joinpath(@__DIR__, "data", "LF10-fragment.tar")
+    @test Tar.list(tarball) == [
+        Tar.Header("LF10/LF10_B.mtx", :file, 0o100600, 367, "")
+    ]
+end