Implement getch() on Linux for musl

This is a workaround for a broken `getpass()` on `musl`, which doesn't
interact with our `with_fake_pty()` properly.

base/util.jl

@@ -257,19 +257,92 @@ graphical interface.
 """
 function getpass end
 
+using Base.BinaryPlatforms: libc, HostPlatform
+
+# Windows and musl require getpass() workarounds based on `getch()`
 if Sys.iswindows()
+    _getch() = UInt8(ccall(:_getch, Cint, ()))
+elseif libc(HostPlatform()) == "musl"
+    struct termios_t
+        c_iflag::Cuint
+        c_oflag::Cuint
+        c_cflag::Cuint
+        c_lflag::Cuint
+        c_line::Cuchar
+        c_cc::NTuple{32,Cuchar}
+        __c_ispeed::Cuint
+        __c_ospeed::Cuint
+
+        function termios_t(d::Dict = Dict())
+            return new(
+                Cuint(get(d, :c_iflag, 0)),
+                Cuint(get(d, :c_oflag, 0)),
+                Cuint(get(d, :c_cflag, 0)),
+                Cuint(get(d, :c_lflag, 0)),
+                Cchar(get(d, :c_line, 0)),
+                ntuple(idx -> Cuchar(get(d, :c_cc, zeros(UInt, 32))[idx]), 32),
+                Cuint(get(d, :__c_ispeed, 0)),
+                Cuint(get(d, :__c_ospeed, 0)),
+            )
+        end
+    end
+
+    function Base.Dict(termios::termios_t)
+        return Dict{Symbol,Any}(
+            :c_iflag => termios.c_iflag,
+            :c_oflag => termios.c_oflag,
+            :c_cflag => termios.c_cflag,
+            :c_lflag => termios.c_lflag,
+            :c_line => termios.c_line,
+            :c_cc => collect(termios.c_cc),
+            :__c_ispeed => termios.__c_ispeed,
+            :__c_ospeed => termios.__c_ospeed,
+        )
+    end
+
+    _tcgetattr(fd, termios) = ccall(:tcgetattr, Cint, (Cint, Ref{termios_t}), fd, termios)
+    _tcsetattr(fd, flags, termios) = ccall(:tcsetattr, Cint, (Cint, Cint, Ref{termios_t}), fd, flags, termios)
+
+    # Taken from musl's `arch/x86_64/bits/termios.h`
+    const ICANON    = Cuint(2)
+    const ECHO      = Cuint(10)
+    const VTIME     = 5
+    const VMIN      = 6
+    const TCSANOW   = 0
+    const TCSADRAIN = 1
+
+    function _getch()
+        old = termios_t()
+        systemerror("tcgetattr", _tcgetattr(0, old) < 0)
+
+        # Switch struct to dict for mutation
+        old = Dict(old)
+        old[:c_lflag] = old[:c_lflag] & ~ICANON
+        old[:c_lflag] = old[:c_lflag] & ~ECHO
+        old[:c_cc][VMIN] = 1
+        old[:c_cc][VTIME] = 0
+        systemerror("tcsetattr", _tcsetattr(0, TCSANOW, termios_t(old)) < 0)
+        val = read(stdin, 1)
+        old[:c_lflag] = old[:c_lflag] | ICANON
+        old[:c_lflag] = old[:c_lflag] | ECHO
+        systemerror("tcsetattr", _tcsetattr(0, TCSADRAIN, termios_t(old)) < 0)
+        return only(val)
+    end
+end
+
+if Sys.iswindows() || libc(HostPlatform()) == "musl"
 function getpass(input::TTY, output::IO, prompt::AbstractString)
     input === stdin || throw(ArgumentError("getpass only works for stdin"))
     print(output, prompt, ": ")
     flush(output)
     s = SecretBuffer()
     plen = 0
     while true
-        c = UInt8(ccall(:_getch, Cint, ()))
+        c = _getch()
         if c == 0xff || c == UInt8('\n') || c == UInt8('\r')
             break # EOF or return
         elseif c == 0x00 || c == 0xe0
-            ccall(:_getch, Cint, ()) # ignore function/arrow keys
+            _getch() # ignore function/arrow keys
         elseif c == UInt8('\b') && plen > 0
             plen -= 1 # delete last character on backspace
         elseif !iscntrl(Char(c)) && plen < 128