Handle tiny files gracefully

Lua detection functions used to cause fatal errors on failure to read.

We could also reconsider treating detection errors as fatal.

plugins/elf.lua

@@ -18,7 +18,7 @@
 -- See man 5 elf, /usr/include/elf.h and /usr/include/llvm/Support/ELF.h
 
 local detect = function (c)
-	return c:read (4) == "\x7FELF"
+	return #c >= 4 and c:read (4) == "\x7FELF"
 end
 
 local ph_type_table = {

plugins/gzip.lua

@@ -16,7 +16,7 @@
 --
 
 local detect = function (c)
-	return c:read (2) == "\x1f\x8b"
+	return #c >= 2 and c:read (2) == "\x1f\x8b"
 end
 
 local function latin1_to_utf8 (s)

plugins/pcap.lua

@@ -16,11 +16,17 @@
 --
 
 local detect = function (c)
+	if #c < 4 then
+		return false
+	end
 	local magic = c:u32 ()
 	return magic == 0xa1b2c3d4 or magic == 0xd4c3b2a1
 end
 
 local detect_ng = function (c)
+	if #c < 8 then
+		return false
+	end
 	local magic = c (9):u32 ()
 	return c:u32 () == 0x0a0d0d0a
 		and (magic == 0x1a2b3c4d or magic == 0x4d3c2b1a)

plugins/pdf.lua

@@ -335,7 +335,7 @@ end
 -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 local detect = function (c)
-	return c:read (5) == "%PDF-"
+	return #c >= 5 and c:read (5) == "%PDF-"
 end
 
 local decode_xref_subsection = function (lex, start, count, result)

plugins/vdi.lua

@@ -16,6 +16,9 @@
 --
 
 local detect = function (c)
+	if #c < 68 then
+		return false
+	end
 	c.position = 65
 	return c:read (4) == "\x7F\x10\xDA\xBE"
 end

plugins/xcursor.lua

@@ -16,7 +16,7 @@
 --
 
 local detect = function (c)
-	return c:read (4) == "Xcur"
+	return #c >= 4 and c:read (4) == "Xcur"
 end
 
 -- https://www.x.org/releases/current/doc/man/man3/Xcursor.3.xhtml