* Fix panic due to invalid regex in a syntax file
When a user's custom syntax file has a malformed filename regex or
header regex, MakeHeaderYaml() returns error but we do not properly
handle it, which results in a panic due to a dereference of the `header`
pointer which is nil:
Micro encountered an error: runtime.errorString runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:221 (0x44c367)
runtime/panic.go:220 (0x44c337)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:709 (0x82bc0f)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:392 (0x828292)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:261 (0x8278c8)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:203 (0x8b9e7b)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:331 (0x8ba9e5)
runtime/proc.go:255 (0x4386a7)
runtime/asm_amd64.s:1581 (0x467941)
* Do not ignore invalid filename regex error in a syntax file
When the filename regex in a syntax file is malformed but the subsequent
header regex is correct, the filename regex error gets silently ignored,
since the `err` value is overwritten by the subsequent successful header
regex result.
This change introduces header files for syntax files. The header
files only contain the filetype and detection info and can be
parsed much faster than parsing a full yaml file. To determine
which filetype a file is, only scanning the headers is necessary
and afterwards only one yaml file needs to be parsed. Use the
make_headers.go file to generate the header files. Micro expects
that all default syntax files will have header files and that
custom user syntax files may or may not have them. Resolving
includes within syntax has not yet been implemented. This
optimization improves startup time.
Ref #1427
