wasi preview 2 support (#4027)

* all: wasip2 support

Co-authored-by: Randy Reddig <randy.reddig@fastly.com>

src/syscall/env_libc.go

@@ -0,0 +1,59 @@
+//go:build darwin || nintendoswitch || wasip1
+
+package syscall
+
+import (
+	"unsafe"
+)
+
+func Environ() []string {
+
+	// This function combines all the environment into a single allocation.
+	// While this optimizes for memory usage and garbage collector
+	// overhead, it does run the risk of potentially pinning a "large"
+	// allocation if a user holds onto a single environment variable or
+	// value.  Having each variable be its own allocation would make the
+	// trade-off in the other direction.
+
+	// calculate total memory required
+	var length uintptr
+	var vars int
+	for environ := libc_environ; *environ != nil; {
+		length += libc_strlen(*environ)
+		vars++
+		environ = (*unsafe.Pointer)(unsafe.Add(unsafe.Pointer(environ), unsafe.Sizeof(environ)))
+	}
+
+	// allocate our backing slice for the strings
+	b := make([]byte, length)
+	// and the slice we're going to return
+	envs := make([]string, 0, vars)
+
+	// loop over the environment again, this time copying over the data to the backing slice
+	for environ := libc_environ; *environ != nil; {
+		length = libc_strlen(*environ)
+		// construct a Go string pointing at the libc-allocated environment variable data
+		var envVar string
+		rawEnvVar := (*struct {
+			ptr    unsafe.Pointer
+			length uintptr
+		})(unsafe.Pointer(&envVar))
+		rawEnvVar.ptr = *environ
+		rawEnvVar.length = length
+		// pull off the number of bytes we need for this environment variable
+		var bs []byte
+		bs, b = b[:length], b[length:]
+		// copy over the bytes to the Go heap
+		copy(bs, envVar)
+		// convert trimmed slice to string
+		s := *(*string)(unsafe.Pointer(&bs))
+		// add s to our list of environment variables
+		envs = append(envs, s)
+		// environ++
+		environ = (*unsafe.Pointer)(unsafe.Add(unsafe.Pointer(environ), unsafe.Sizeof(environ)))
+	}
+	return envs
+}
+
+//go:extern environ
+var libc_environ *unsafe.Pointer