Don't check magic bytes on save (fixes #267)

Project.toml

@@ -14,4 +14,4 @@ Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["FilePathsBase", "Test", "Random"]
+test = ["FilePathsBase", "Random", "Test"]

src/loadsave.jl

@@ -114,9 +114,12 @@ savestreaming
 
 # if a bare filename or IO stream are given, query for the format and dispatch
 # to the formatted handlers below
-for fn in (:load, :loadstreaming, :save, :savestreaming, :metadata)
+for fn in (:load, :loadstreaming, :metadata)
     @eval $fn(file, args...; options...) = $fn(query(file), args...; options...)
 end
+for fn in (:save, :savestreaming)
+    @eval $fn(file, args...; options...) = $fn(query(file; checkfile=false), args...; options...)
+end
 
 # return a save function, so you can do `thing_to_save |> save("filename.ext")`
 save(file; options...) = data -> save(file, data; options...)

src/query.jl

@@ -62,24 +62,29 @@ end
 
 
 """
-`query(filename)` returns a `File` object with information about the
+    query(filename; checkfile=true)
+
+Return a `File` object with information about the
 format inferred from the file's extension and/or magic bytes.
+If `filename` already exists, the file's magic bytes will take priority
+unless `checkfile` is false.
 """
-function query(filename)
+function query(filename; checkfile::Bool=true)
+    checkfile &= isfile(filename)
     _, ext = splitext(filename)
     if haskey(ext2sym, ext)
         sym = ext2sym[ext]
         no_magic = !hasmagic(sym)
-        if lensym(sym) == 1 && (no_magic || !isfile(filename)) # we only found one candidate and there is no magic bytes, or no file, trust the extension
+        if lensym(sym) == 1 && (no_magic || !checkfile) # we only found one candidate and there is no magic bytes, or no file, trust the extension
             return File{DataFormat{sym}}(filename)
-        elseif !isfile(filename) && lensym(sym) > 1
+        elseif !checkfile && lensym(sym) > 1
             return File{DataFormat{sym[1]}}(filename)
         end
         if no_magic && !hasfunction(sym)
             error("Some formats with extension ", ext, " have no magic bytes; use `File{format\"FMT\"}(filename)` to resolve the ambiguity.")
         end
     end
-    !isfile(filename) && return File{unknown_df}(filename) # (no extension || no magic byte) && no file
+    !checkfile && return File{unknown_df}(filename) # (no extension || no magic byte) && no file
     # Otherwise, check the magic bytes
     file!(query(open(filename), abspath(filename)))
 end

test/loadsave.jl

@@ -292,6 +292,19 @@ end # module Dummy
     @test_throws Exception save("missing.fmt",5)
 end
 
+@testset "Overwrite file with bad magic bytes" begin
+    # issue #267
+    a = [0x01,0x02,0x03]
+    fn = tempname()*".dmy"
+    open(fn, "w") do io
+        write(io, "Ceci n'est pas un DUMMY")
+    end
+    save(fn, a)
+    @test isa(query(fn), File{format"DUMMY"})
+    @test load(fn) == a
+    rm(fn)
+end
+
 del_format(format"DUMMY")
 
 # PPM/PBM can be either binary or text. Test that the defaults work,