What is the test for a prison official to be liable under the Eighth Amendment for failing to protect an inmate from harm?

"The Constitution does not mandate comfortable prisons, but neither does it permit inhumane ones." Farmer v. Brennan, 511 U.S. 825, 832 (1994) (internal quotation marks and citation omitted). "[A] prison official violates the Eighth Amendment only when two requirements are met. First, the deprivation alleged must be, objectively, sufficiently serious, a prison official's act or omission must result in the denial of the minimal civilized measure of life's necessities." Id. at 834 (internal quotation marks and citations omitted). Second, the prison official must subjectively have a sufficiently culpable state of mind, "one of deliberate indifference to inmate health or safety." Id. (internal quotation marks and citations omitted). The official is not liable under the Eighth Amendment unless he "knows of and disregards an excessive risk to inmate health or safety; the official must both be aware of facts from which the inference could be drawn that a substantial risk of serious harm exists, and he must also draw the inference." Id. at 837. Then he must fail to take reasonable measures to abate the substantial risk of serious harm. Id. at 847. Mere negligent failure to protect an inmate from harm is not actionable under 1983. Id. at 835.